GitLab

And a few new accessors for Path that are helpful and reasonable.

As discussed on the issue thread, ipld.Kind and schema.TypeKind are more
intuitive, closer to the spec wording, and just generally better in the
long run.

The changes are almost entirely automated via the commands below. Very
minor changes were needed in some of the generators, and then gofmt.

	sed -ri 's/\<Kind\(\)/TypeKind()/g' **/*.go
	git checkout fluent # since it uses reflect.Value.Kind

	sed -ri 's/\<Kind_/TypeKind_/g' **/*.go
	sed -i 's/\<Kind\>/TypeKind/g' **/*.go
	sed -i 's/ReprKind/Kind/g' **/*.go

Plus manually undoing a few renames, as per Eric's review.

Fixes #94.

Hopefully this increases clarity and eases comprehension.

Notes and discussion can be found at
https://github.com/ipld/go-ipld-prime/issues/54 (and also
I suppose in some of our weekly video chats, but I'd have
to go on quite a dig to find the relevant links and time).

Many many refernces to 'ns' are also updated to 'np',
making the line count in this diff pretty wild.

See the changelog for discussion; this had already been on the docket
for a while now.

Walk functions weren't sufficiently fleshed out.

A few notes about expansion effects due to paths are now made.

General consistency pass across all the methods.

This is a *lot* of changes.  It's the most significant change to date,
both in semantics and in character count, since the start of this repo.
It changes the most central interfaces, and significantly so.

But all tests pass.  And all benchmarks are *improved*.

The Node interface (the reading side) is mostly unchanged -- a lot of
consuming code will still compile and work just fine without changes --
but any other Node implementations out there might need some updating.

The NodeBuilder interface (the writing side) is *extremely* changed --
any implementations out there will *definitely* need change -- and most
consumers will too.  It's unavoidable with a semantic fix this big.
The performance improvements should make it worth your while, though.

If you want more background on how and why we got here, you've got
quite a few commits on the "research-admissions" branches to catch up
on reading.  But here's a rundown of the changes:

(Get a glass of water or something calming before reading...)

=== NodeAssembler introduced! ===

NodeAssembler is a new interface that describes most of the work of
creating and filling data into a new Node.

The NodeBuilder interface is still around, but changed in role.
A NodeBuilder is now always also a NodeAssembler; additionally, it can
return the final Node to you.

A NodeAssembler, unlike NodeBuilder, can **not** return a Node to you.
In this way, a NodeBuilder represents the ability to allocate memory.
A NodeAssembler often *does not*: it's just *filling in* memory.

This design overall is much more friendly to efficient operations:
in this model, we do allocations in bulk when a NodeBuilder is used,
and then NodeAssemblers are used thereafter to fill it in -- this
mental model is very friendly to amortizing memory allocations.
Previously, the NodeBuilder interface made such a pattern of use
somewhere between difficult and outright impossible, because it was
modeled around building small values, then creating a bigger value and
inserting the smaller ones into it.

This is the key change that cascaded into producing the entire other
set of changes which land in this commit.

The NodeBuilder methods for getting "child builders" are also gone
as a result of these changes.  The result feels a lot smoother.
(You can still ask for the NodeStyle for children of a recursive kind!
But you'll find that even though it's possible, it's rarely necessary.)

We see some direct improvements from this interface change already.
We'll see even more in the future: creating values when using codegen'd
implementations of Node was hugely encumbered by the old NodeBuilder
model; NodeAssembler *radically* raises the possible ceiling for
performance of codegen Node implementations.

=== NodeStyle introduced ===

NodeStyle is a new interface type that is used to carry information
about concrete node implementations.

You can always use a NodeStyle to get a NodeBuilder.

NodeStyle may also have additional features on it which can be detected
by interface checks.  (This isn't heavily used yet, but we imagine it
might become handy in the future.)

NodeStyle replaces NodeBuilder in many function arguments,
because often what we wanted was to communicate a selection of Node
implementation strategy, but not actually the start of construction;
the NodeStyle interface now allows us to *say that*.

NodeStyle typically cost nothing to pass around, whereas a NodeBuilder
generally requires an allocation to create and initialize.  This means
we can use NodeStyle more freely in many contexts.

=== node package paths changed ===

Node implementations are now in packages under the "node/*" directory.
Previously, they were under an "impl/*" directory.

The "impl/free" package is replaced by the the "node/basic" package!
The package name was "ipldfree"; it's now "basicnode".

=== basicnode is an improved runtime/anycontent Node implementation ===

The `basicnode` package works much the same as the `ipldfree` package
used to -- you can store any kind of data in it, and it just does as
best it can to represent and handle that, and it works without any
kind of type info nor needs of compile-time special support, etc --
while being just quietly *better at it*.

The resident memory size of most things has gone down.
(We're not using "fat unions" in the implementation anymore.)

The cost of iterating maps has gone down *dramatically*.
Iteration previously suffered from O(n) allocations due to
expensive `runtime.conv*` calls when yielding keys.
Iteration is now O(1) (!!) because we redesigned `basicnode` internals
to use "internal pointers" more heavily, and this avoids the costs
from `runtime.conv*`.
(We could've done this separately from the NodeAssembler change,
admittedly.  But both are the product of research into how impactful
clever use of "internal pointers" can be, and lots of code in the
neighborhood had to be rewritten for the NodeAssembler interface,
so, these diffs arrive as one.)

Error messages are more informative.

Many small operations should get a few nanoseconds faster.
(The implementation uses more concrete types and fewer switch
statements.  The difference probably isn't the most noticeable part of
all these changes, but it's there.)

--- basicnode constructor helpers do all return pointers ---

All the "New*" helper functions in the basicnode package return
interfaces which are filled by a pointer now.
This is change from how they worked previously when they were first
implemented in the "rsrch" package.

The experience of integrating basicnode with the tests in the traversal
package made it clear that having a mixture of pointer and non-pointer
values flying around will be irritating in practice.  And since it is
the case that when returning values from inside a larger structure,
we *must* end up returning a pointer, pointers are thus what we
standardize on.

(There was even some writeup in the HACKME file about how we *might*
encounter issues on this, and need to change to pointers-everywhere --
the "pointer-vs-value inhabitant consistency" heading.  Yep: we did.
And since this detail is now resolved, that doc section is dropped.)

This doesn't really make any difference to performance.
The old way would cause an alloc in those method via 'conv*' methods;
the new way just makes it more explicit and go through a different
runtime method at the bottom, but it's still the same number of
allocations for essentially the same reasons.  (I do wonder if at some
future point, the golang compiler might get cleverer about eliding
'conv*' calls, and then this change we make here might be unfortunate;
but that's certainly not true today, nor in the future at any proximity
that I can foresee.)

=== iterator getters return nil for wrong-kind ===

The Node.MapIterator and Node.ListIterator methods now return nil
if you call them on non-maps or non-lists.

Previously, they would return an iterator, but using it would just
constantly error.

I don't think anyone was honestly really checking those error thunks,
and they made a lot of boilerplate white noise in the implementations,
and the error is still entirely avoidable by checking the node kind
up-front (and this is strictly preferable anyway, since it's faster
than getting an error thunk, poking it to get the error, etc)...
so, in total, there seem like very few reasons these were useful:
the idea is thus dropped.

Docs in the Node interface reflect this.

=== node/mixins makes new Node implementations easier ===

The mixins package isn't usable directly, but if you're going to make
a new Node implementation, it should save you a lot of typing...
and also, boost consistency of basic error handling.

Codegen will look forward to using this.  (Codegen already had much of
these semantics internally, and so this package is sort of lifting that
back out to be more generally usable.  By making it live out here as
exported symbols in the core library, we should also reduce the sheer
character count of codegen output.)

=== 'typed.Node' is now 'schema.TypedNode' ===

A bunch of interfaces that were under the "impl/typed" path moved to
be in the "schema" package instead.  This probably makes sense to you
if you look at them and needs no further explanation.

(The reason it comes in this diff, though, is that it was forced:
adding better tests to the traversal package highlighted a bunch of
cyclic dependency issues that came from 'typed.Node' being in a
package that had concrete use of 'basicnode'.)

=== codecs ===

The 'encoding' package is now named 'codec'.

This name is shorter; it's more in line with vocabulary we use
elsewhere in the IPLD project (whereas 'encoding' was more of a nod
to the naming found in the golang standard library); and in my personal
opinion it does better at describing the both directions of the process
(whereas 'encoding' sounds like only the to-linear-bytes direction).

I just like it better.

=== unmarshal functions no longer return node ===

Unmarshal functions accept an NodeAssembler parameter (rather than
a NodeBuilder, as before, nor a NodeStyle, which might also make sense
in the new family of interfaces).

This means they no longer need to return a Node, either -- the caller
can decide where the unmarshalled data lands.  If the caller is using
a NodeBuilder, it means they can call Build on that to get the value.
(If it's a codegen NodeBuilder with More Information, the caller can
use any specialized functions to get the more informative pointers
without need for casting!)

Broadly speaking, this means users of unmarshal functions have more
control over how memory allocation comes into play.

We may want to add more helper functions to the various codec packages
which take a NodeStyle argument and do return a Node.  That's not in
this diff, though.  (Need to decide what pattern of naming these
various APIs would deserve, among other things.)

=== the fluent package ===

The fluent package changed significantly.

The readonly/Node side of it is dropped.  It didn't seem to get a ton
of exercise in practice; the 'traversal' package (and in the future,
perhaps also a 'cursor' package) addresses a lot of the same needs,
and what remains is also covered well these days by the 'must' package;
and the performance cost of fluent node wrappers as well as the
composability obstruction of them... is just too much to be worth it.

The few things that used fluent.Node for reading data now mostly use
the 'must' package instead (and look better for it, imo).

It's possible that some sort of fluent.Node will be rebuilt someday,
but it's not entirely clear to me what it should look like, and indeed
whether or not it's a good idea to have in the repo at all if the
performance of it is counterindicated in a majority of situations...
so, it's not part of today's update.

The writing/NodeBuilder/NodeAssembler fluent wrappers are continued.
It's similar to before (panics promptly on errors, and has a lot of
closures)... but also reflects all of the changes made in the migration
towards NodeAssembler: it doesn't return intermediate nodes, and
there's much less kerfuffle with getting child builders.
Overall, the fluent builders are now even more streamlined than before;
the closures need even fewer parameters; great success!

The fluent.NodeAssembler interface retains the "Create" terminology
around maps and lists, even though in the core interfaces,
the ipld.NodeAssembler interface now says "Begin" for maps and lists.
This is because the fluent.NodeAssembler approach, with its use of
closures, really does do the whole operation in one swoop.

(It's amusing to note that this change includes finally nuking some
fairly old "REVIEW" comment blocks from the old fluent package which
regarded the "knb" value and other such sadness around typed recursion.
Indeed, we've finally reviewed that: and the answer was indeed to do
something drastically different to make those recursions dance well.)

=== selectors ===

Selectors essentially didn't change as part of this diff.  Neat.

(They should get a lot faster when applied, because our node
implementations hit a lot less interface boxing in common operations!
But the selector code itself didn't need to change to get the gains.)

The 'selector/builder' helper package *did* change a bit.
The changes are mostly invisible to the user.
I do have some questions about the performance of the result; I've got
a sneaking suspicion there's now a bunch of improvements that might be
easier to get to now than they would've been previously.  But, this is
not my quest today.  Perhaps it will deserve some review in the future.

The 'selector/builder' package should be noted as having some
interesting error handling strategies.  Namely, it doesn't.
Any panics raised by the fluent package will just keep rising; there's
no place where they're converted to regular error value returns.
I'm not sure this is a good interface, but it's the way it was before
I started passing through, so that's the way it stays after this patch.

ExploreFieldsSpecBuilder.Delete disappears.  I hope no one misses it.
I don't think anyone will.  I suspect it was there only because the
ipld.MapBuilder interface had such a method and it seemed like a
reasonable conservative choice at the time to proxy it; now that the
method proxied is gone, though, so too shall go this.

=== traversal ===

Traversal is mostly the same, but a few pieces of config have new names.

`traversal.Config.LinkNodeBuilderChooser` is now
`traversal.Config.LinkTargetNodeStyleChooser`.
Still a mouthful; slightly more accurate; and reflects that it now
works in terms of NodeStyle, which gives us a little more finesse in
reasoning about where NodeBuilders are actually created, and thus
better control and insight into where allocations happen.

`traversal.NodeBuilderChooser` is now
`traversal.LinkTargetNodeStyleChooser` for the same reasons.

The actual type of the `LinkTargetNodeStyleChooser` now requires
returning a `NodeStyle`, in case all the naming hasn't made it obvious.

=== disappearing node packages ===

A couple of packages under 'impl/*' are just dropped.

This is no real loss.  The packages dropped were Node implementations
that simply weren't done.  Deleting them is an increase in honesty.

This doesn't mean something with the same intentions as those packages
won't come back; it's just not today.

--- runtime typed node wrapper disappeared ---

This one will come back.  It was just too much of a pain to carry
along in this diff.  Since it was also a fairly unfinished
proof-of-concept with no downstream users, it's easier to drop and
later reincarnate it than it is to carry it along now.

=== linking ===

Link.Load now takes a `NodeAssembler` parameter instead of a
`NodeBuilder`, and no longer returns a `Node`!

This should result in callers having a little more control over where
allocations may occur, letting them potentially reuse builders, etc.

This change should also make sense considering how codec.Unmarshal
now similarly takes a NodeAssembler argument and does not return
a Node value since its understood that the caller has some way to
access or gather the effects, and it's none of our business.

Something about the Link interface still feels a bit contorted.
Having to give the Load method a Loader that takes half the same
arguments all over again is definitely odd.  And it's tempting to take
a peek at fixing this, since the method is getting a signature change.
It's unclear what exactly to do about this, though, and probably
a consequential design decision space... so it shall not be reopened
today during this other large refactor.  Maybe soon.  Maybe.

=== the dag-json codec ===

The dag-json codec got harder to implement.  Rrgh.

Since we can't tell if something is going to become a Link until
*several tokens in*, dag-json is always a bit annoying to deal with.
Previously, however, dag-json could still start optimistically building
a map node, and then just... quietly drop it if we turn out to be
dealing with a link instead.  *That's no longer possible*: the process
of using NodeAssembler doesn't have a general purpose mechanism for
backtracking.

So.  Now the dag-json codec has to do even more custom work to buffer
tokens until it knows what to do with them.  Yey.

The upside is: of course, the result is actually faster, and does fewer
memory allocations, since it gathers enough information to decide what
it's doing before it begins to do it.
(This is a lovely example of the disciplined design of NodeAssembler's
interface forcing other code to be better behaved and disciplined!)

=== traversal is faster ===

The `BenchmarkSpec_Walk_MapNStrMap3StrInt/n=32` test has about doubled
in speed on the new `basicnode` implementation in comparison to the old
`ipldfree.Node` implementation.

This is derived primarily from the drop in costs of iteration on
`basicnode` compared to the old `ipldfree.Node` implementation.

Some back-of-the-envelope math on the allocation still left around
suggest it could double in speed again.  The next thing to target
would be allocations of paths, followed by iterator allocations.
Both are a tad trickier, though (see a recently merge-ignore'd
commit for notes on iterators; and paths... paths will be a doozy
because the path forward almost certainly involves path values
becoming invalid if retained beyond a scope, which is... unsafe),
so certainly need their own efforts and separate commits.

=== marshalling is faster ===

Marshalling is much faster on the new `basicnode` implementation in
comparison to the old `ipldfree.Node` implementation.
Same reasons as traversal.

Some fixes to marshalling which previously caused unnecessary
allocations of token objects during recursions have also been made.
These improve speed a bit (though it's not nearly as noticeable as the
boost provided by the Node implementation improvements to iteration).

=== size hints showed up all over the place ===

The appearance of size hint arguments to assembly of maps and lists
is of course inevitable from the new NodeAssembler interface.

It's particularly interesting to see how many of them showed up in
the selector and selectorbuilder packages as constants.

And super especially interesting how many of them are very small
constants.  44 zeros.  86 ones.  25 twos.  9 threes.  2 fours.
(Counted via variations of `grep -r 'Map(.*4, func' | wc -l`.)
It's quite a distribution, neh?  We should probably consider some
more optimizations specifically targeted to small maps.
(This is an unscientific sample, and shifted by what we chose to
focus on in testing, etc etc, but the general point stands.)

`-1` is used to indicate "no idea" for size.  There's a small fix
to the basicnode implementations to allow this.  A zero would work
just as well in practice, but using a negative number as a hint to
the human seems potentially useful.  It's a shame we can't make the
argument optional; oh well.

=== codegen ===

The codegen packages still all compile... but do nonsensical things,
for the moment: they've not been updated to emit NodeAssembler.

Since the output of codegen still isn't well rigged to test harnesses,
this breakage is silent.

The codegen packages will probably undergo a fairly tabula-rasa sweep
in the near future.  There's been a lot of lessons learned since the
start of the code currently there.  Updating to emit the NodeAssembler
interface will be such a large endeavor it probably represents a good
point to just do a fresh pass on the whole thing all at once.

--------

... and that's all!

Fun reading, eh?

Please do forgive the refactors necessary for all this.  Truly, the
performance improvements should make it all worth your while.

traversal.NodeBuilderChooser can now return error.  Also, the default no longer returns ipldfree.NodeBuilder.

The traversal package having a dependency on the ipldfree Node
implementation package was problematic: it's important that we be able
to benchmark traversal *in combination* with different implementations
of Node, and it turns out if we want to build reusable test and
benchmark functions for that, we get cyclic dependencies when then
trying to use them on our most common implementation!  Ouch.

Having a default implementation of Node referenced was originally out
of a desire for convenience.  But it's logically dubious anyway.
Even the convenience gain in practice is questionable since one still
always needs to configure a LinkLoader in the same areas.  At any rate,
hopefully the error message is helpful enough to make this low-impact.

(We could probably benefit from more helper methods around this, too.
But we can review that later.  Preferably after the 'NodeStyle' feature
from the R&D branches lands, also, which should make this whole area
better named, clearer about where its allocations arrive, and just
less fidgety in general.)

ipld.Path is now a slice of ipld.PathSegment instead of strings.

This should be an improvement in sanity: there are now several fewer
places importing "strconv", and that's just always a good thing.

We will also be free in the future to add PathSegment-based accessor
methods to ipld.Node, as has already been commented as a desire;
and, to use PathSegment in building better typed errors
(which is the specific thing that provokes this diff today and now).

The implementation of PathSegment is now also based on a struct-style
union rather than an interface style union.  There are comments about
this in the diff.  I do not wish to comment on how much time I've spent
looking at golang assembler and runtime internals while trying to find
a path to a more perfect compromise between ergonomics and performance.
tl;dr Selectors will probably get faster and trigger fewer allocations;
ipld.Path will probably take slightly more memory (another word per
path segment), but not enough to care about for any practical purposes.

I did not attempt to hoist the SegmentIterator features from the
selector package to anywhere more central.
It may be a fine idea to do so someday; I just don't presently have
a formed opinion and am not budgeting time to consider it today.
Signed-off-by: Eric Myhre <hash@exultant.us>

"tp" makes a lot less sense now that we fixed the stutter in the
previous name of this struct (TraversalProgress).  "prog" will do.
Signed-off-by: Eric Myhre <hash@exultant.us>

No semantics change; this is a "sed refactor" in complexity.

Literally:

- `s/TraversalReason/VisitReason/g`
- `s/TraversalProgress/Progress/g`
- `s/TraversalConfig/Config/g`
- `s/Traverse(/WalkMatching(/g`
- `s/TraverseInformatively(/WalkAdv(/g`
- `s/TraverseTransform/WalkTransforming/g`

There's a few more internally, but the above covers all of the public
exported interface changes.

These changes generally reduce stutter
(e.g. previously traversal.TraversalProgress was quite a gulp),
and mostly result in shorter function names and signatures overall
(and the signatures are particularly noticable, since VisitFuncs are
often actually declared inline by calling code).

A few docs strings also received manual touch-up.
Signed-off-by: Eric Myhre <hash@exultant.us>

Most important things first!  To follow this refactor:

```
sed s/TraverseField/LookupString/g
sed s/TraverseIndex/LookupIndex/g
```

It is *literally* a sed-refactor in complexity.

---

Now, details.

This has been pending for a while, and there is some discussion in
https://github.com/ipld/go-ipld-prime/issues/22 .

In short, "Traversal" seemed like a mouthful;
"Field" was always a misnomer here;
and we've discovered several other methods that we *should* have
in the area as well, which necessitated a thought about placement.

In this commit, only the renames are applied, but you can also see
the outlines of two new methods in the Node interface, as comments.
These will be added in future commits.
Signed-off-by: Eric Myhre <hash@exultant.us>

And test.
Signed-off-by: Eric Myhre <hash@exultant.us>

Autocomplete on TraversalProgress was suggesting *way* too many things.
It's now reigned in.

Being able to say `tp.Ctx` is gone, but that's a small price to pay.
(And perhaps we should actually move the Context up to TP?  Wouldn't
seem unreasonable.  I think the main reason it's in TC right now is
to reduce the size of memory that's shuffled when TP is passed by value
down the stack, and it's unclear how consequential that's really
going to be in the big picture.  We can review this later.)
Signed-off-by: Eric Myhre <hash@exultant.us>

NodeBuilderChooser returned -- sorry.  There are some cases where it's
unavoidable.  (If we have schemas / typed nodes, we can do feature
detection on the node to see if it can recommend an appropriate
NodeBuilder for the far side of its link.  If we're fine using Node
implementations that have 'any' storage support, it's not an issue.
If we're using *bind* implementation Nodes -- which are constrained
by the Golang native type system, and yet not perfectly mapped onto
our own typed.Node constraint declarations -- then we need some way
to choose the NodeBuilder.  So.  NodeBuilderChooser.  Again: sorry.)

We can probably still expect the NodeBuilderChooser to be fixed
(probably returning one of the 'any' storage supporting Node impls)
in practice, since these traversal systems are generally for either
"low context" usage (e.g., dunno what the data is) or will be used
on typed/schema nodes, in which case we're again free of these issues.
And the default values will indeed do this, using our new helpful
default initialization for TraversalConfig.

TraversalConfig will now always be initialized to sensible default
values when using any of the traversal methods.  That means you can
always assume `tp.Ctx`, etc, are going to be set -- no nil checks
necssary -- even though they're optional to the caller.

Error messages from traversal.Focus are touched up a bit for both
consistency and clearer information.  There's more work to do here:
we want these to be full-on typed errors before we call it "done".
This work will probably begin soon, but be spread over quite a few
commits (we also need to go back up to the *main* ipld package and
make types for Node-level errors, and make sure tests exist to
standardize those errors across Node implementations as well).
Signed-off-by: Eric Myhre <hash@exultant.us>

Having a function called "Kind" return a "ReprKind" was inconsistent.

Also, we want to introduce a "Kind" method on `typed.Node` in the future.

No logical content to this change: you can safely refactor with sed.
Signed-off-by: Eric Myhre <hash@exultant.us>

Yes, this one has moved about a bunch now.  Hopefully this is the last time.
It's true and elegant that paths really only emerge as descriptions of traversal
progress; however, this misses a few other practicalities.
There are other kinds of traversal (other than the traversal package, whoa!) out
there: see the typesystem packages, which had grown a custom path implementation
simply for error reporting messages!
In general, we seem to want to have Path around for logging and errors, which
will make it increasingly desirable to have available in the root package when
we begin to clean up towards strongly typed errors.
And we also need Path for LinkContext -- and wherever that comes to rest, it
definitely need to not be an import cycle problem, which it *is* if Path is
in traversal and we wanted LinkContext to be *anywhere* else.
All of these point to moving Path back up to the root, and the errors concern
in particular cinches it.

Drop the Path.traverse method.  As has been noted in its comment for a while
now, that method wasn't useful for much, having been replaced by features
in the traversal package.

(Also drop the tests specific to the Path.traverse method.  We should write
more tests against the features now implemented by travesral.Focus... but at
this point, it'd be easier to start over.  The tests we're dropping are against
a different model of traversal (returns rather than visitors) and are also
built against the old hacky ipldfree mutable model which is deprecated and
soon to be dropped.)

Also, a small docs fix: drop description of this Path implementation as a
"merklepath" -- it is not.  These paths are all relative, and do not contain
an innate understanding of hashed object identifiers at their first segment.
Signed-off-by: Eric Myhre <hash@exultant.us>

All traversal/transform/selector/path code to date got a rethink.

For the longest time, I was trying to keep all the traversal code in
the main root IPLD package, thinking/hoping that the number of relevant
functions would be minimal enough that this would be fine.

Somewhere around the time where I conceded that yes, we *do* need
separate methods for writable and readonly paths (it's true that
readonly is a degenerate case of writable -- but they have different
performance characteristics!), I gave up: the number of functions in
play is too high to heap alltogether in the root package namespace.

This leads to a number of other surprising results: namely, that
ipld.Path *isn't* -- it's actually traversal.Path!  We *don't use*
paths anywhere else except as guidance to some forms of traversal,
and logs and progress markers during traversal.  Huh!
Despite being initially surprising, I now regard this as a *good*
code smell.

ipld.Traversal as an interface and ipld.Path.Traverse are dropped.
These turned out to be the wrong level of abstraction:
it's both missing things, and not able to return enough things.
By the time we would fix both, it's not an abstraction anymore.

Path.Traverse didn't know how to use a link loader func -- and
that's correct; it shouldn't -- and it also didn't retain and
return a stack of Nodes it traversed -- which again, it shouldn't,
because in general that would be useless overkill and resource waste...
it's just that both these things are essential in some use cases.
So!  Cut that gordian knot.

The Focus/FocusTransform/Traverse/TraverseTransform methods will now
do all recursion work themselves, internally.  The transform family
will keep stacks of nodes encountered along the way, to aid in the
(presumably) coming tree re-build; the focus family won't.
All of these methods will be able to support link loading, though
implementation of that is still upcoming.

There's a few more fields in TraversalProgress, also preparing for when
we have automatic link loading and traversal: we'll want to be able
to inform the user's callbacks about load edges.  (I'm not a fan of the
term 'block' here, but running with it for the moment.)

TraversalConfig is expected to grow.  Namely, to include link
loaders... and subsequently, probably block writers, as well!  This is
a big subject, though.  It'll probably take several commits and
possibly more than one iteration over time to hammer out a good API.

One thing that's interesting to note that neither TraversalConfig nor
TraversalProgress will grow to contain certain things: for example,
a map of "seen" CIDs when doing link traversal.  Why?  Well, if you've
been mentally modelling things as a graph, this is surprising; surely
any graph traversal needs to remember a set of already-visited nodes.
The trick is... we're *not* doing a graph traversal -- not exactly!
Since our visitor gets a (Node,Path) *tuple*... it would be wrong to
memoize on anything other than the complete tuple; and the Path part
of the tuple means we're actually doing a *tree* walk rather than
a graph walk.  (And of course, since we're operating on DAGs,
termination guarantees are already a non-issue.)  Tada; no "seen" set.
Signed-off-by: Eric Myhre <hash@exultant.us>