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.

This should be more intuitive to Go programmers, since assignments are
generally trivial operations, but conversions imply that extra work
might be needed to adapt the value to fit in the recipient.

The entire change is just:

	sed -ri 's/AssignNode/ConvertFrom/g' **/*.go

Downstream users can very likely use the same line to fix their function
declarations and calls.

Fixes #95.

We only supported representing Int nodes as Go's "int" builtin type.
This is fine on 64-bit, but on 32-bit, it limited those node values to
just 32 bits. This is a problem in practice, because it's reasonable to
want more than 32 bits for integers.

Moreover, this meant that IPLD would change behavior if built for a
32-bit platform; it would not be able to decode large integers, for
example, when in fact that was just a software limitation that 64-bit
builds did not have.

To fix this problem, consistently use int64 for AsInt and AssignInt.

A lot more functions are part of this rewrite as well; mainly, those
revolving around collections and iterating. Some might never need more
than 32 bits in practice, but consistency and portability is preferred.
Moreover, many are interfaces, and we want IPLD interfaces to be
flexible, which will be important for ADLs.

Below are some GNU sed lines which can be used to quickly update
function signatures to use int64:

	sed -ri 's/(func.* AsInt.*)\<int\>/\1int64/g' **/*.go
	sed -ri 's/(func.* AssignInt.*)\<int\>/\1int64/g' **/*.go
	sed -ri 's/(func.* Length.*)\<int\>/\1int64/g' **/*.go
	sed -ri 's/(func.* LookupByIndex.*)\<int\>/\1int64/g' **/*.go
	sed -ri 's/(func.* Next.*)\<int\>/\1int64/g' **/*.go
	sed -ri 's/(func.* ValuePrototype.*)\<int\>/\1int64/g' **/*.go

Note that the function bodies, as well as the code that calls said
functions, may need to be manually updated with the integer type change.
That cannot be automated, because it's possible that an automated fix
would silently introduce potential overflows not being handled.

Some TODOs and FIXMEs for overflow checks are removed, since we remove
some now unnecessary int64->int conversions. On the other hand, the
older codecs based on refmt need to gain some overflow check TODOs,
since refmt uses ints. That is okay for now, since we'll phase out refmt
pretty soon.

While at it, update codectools to use int64 for token Length fields, so
that it properly supports full IPLD integers without machine-dependent
behavior and overflow checks. The budget integer is also updated to be
int64, since the lengths it uses are now int64.

Note that this refactor needed changes to the Go code generator as well
as some of the tests, for the purpose of updating all the code.

Finally, note that the code-generated iterator structs do not use int64
fields internally, even though they must return int64 numbers to
implement the interface. This is because they use the numeric fields to
count up to a small finite amount (such as the number of fields in a Go
struct), or up to the length of a map/slice. Neither of them can ever
outgrow "int".

Fixes #124.

Buffers are not a good option for tests if the other side expects a
reader. Otherwise, the code being tested could build assumptions around
the reader stream being a single contiguous chunk of bytes, such as:

	_ = r.(*bytes.Buffer).Bytes()

This kind of hack might seem unlikely, but it's an easy mistake to make,
especially with APIs like fmt which automatically call String methods.

With bytes.Reader and strings.Reader, the types are much more
restricted, so the tests need to be more faithful.

This doesn't add much in the way of coverage, since it's internally
quickly convergenced with what the Focus tests cover, but in testing,
explicitness is good.

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.

Saying "Node" again in the middle is just purely redundant and doesn't
improve clarity in any meaningful way.  Begone with it.

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.

This is the last part of the cleanup and cyclic import fix started
in the previous commit.

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.)

Add the concept of a suggested node builder to links that are a reference to a specific typed node
that has a code-gen'd node builder

Refactors the limit data on recursive selectors to be a union type

switch maxDepth on ExploreRecursive to union limit type per spec

Fix traversal internals to use it (!) rather than converting segments
to strings, which was both wasteful, and in some cases, *wrong* (!)
(although by coincidence happened to mostly work at present because of
another thing from early-days code that was also technically wrong).

Fix ipldfree.Node to reject LookupString if used on a list node!
(This is the other "wrong" thing that made the traversal coincidentally
work.)

LookupSegment method generation also added to codegen.
Signed-off-by: Eric Myhre <hash@exultant.us>

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>

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>

Fixes a bug where ExploreRecursive would not properly replace ExploreRecursiveEdge (and cause a
panic when traversed) if ExploreUnion was part of the recursion tree.

Properly record last block traversed

refactoring functions in traverseInformatively to simply and optimize logic and also minimize inner
loop function calls

Improve the traverse function so that it only explores relevant fields for low cardinality selectors
and traverses high cardinality selectors for lists correctly

Add link loading to traverse informatively so that you can traverse across links

Extract the selector buidler to a seperate package

Adds a builder class to quickly assemble selectors nodes, using a DSL that enforces the underlying
schema. This allows quick assembly of selector nodes. This will likely be removed once IPLD Schema
code gen is finished, but for the time being, it provides a quick way to ensure mostly typesafe
selector nodes.

Rename SelectorContext to ParsedParent (more accurate) and put in a ParseContext state object which
is tracked w/o var args

Support validating recursive selectors to match ExploreRecursive with ExploreRecursiveEdge

An initial implementation of recursive selectors that offers a way to traverse them. Not covered is
validating ExploreRecursiveEdge being under an ExploreRecursive selector

update comment to clearly indicate return value correctly