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

(Mostly; all the scalars work; the composites are todo's.)

Heading in the direction of being able to construct stuff for testing.
Almost there.
Signed-off-by: Eric Myhre <hash@exultant.us>

Not sure why these weren't there already.

(Probably because of the ongoing discussion about "is link a kind at
the data model level?" -- to which I still opine "ideally, no".  But
that's something we're probably going to concede "yes" on *for now*,
and try to straighten out later once we get something ready for the
higher level type systems.)
Signed-off-by: Eric Myhre <hash@exultant.us>

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

Sort float and bytes (less ubiquitous things) to the bottom.

Drop mention of uint from the ipldfree implementation.  So far all spec
discussions have tended towards mentioning "integers" as a type, and
definitely not "unsigned integers" or "positive-only integers" as a
distinct kind in the core Data Model.
Signed-off-by: Eric Myhre <hash@exultant.us>

That subject has more to do with SerializableNode.  Which also might
not be the name we end up with for that.  ID'able Node?  LinkableNode?
Signed-off-by: Eric Myhre <hash@exultant.us>

The very first draft tried to get away with *one* "Kind" enumeration,
but that quickly became odd and shakey; *two* separate "Kind" enums
(one for the Data Model, one for the lower level Representation;
working terms, and mine) fits a lot better.  The latter is what we're
committing here.

Also of interest here is a proposal for a distinction between whether
fields are *required* vs *nullable*.  I'm not sure this has been done
before in any of the other systems I've examined so far; it's a concept
I think we'll want for dealing with the subtle distinction between
whether some piece of data *matches* our schema vs whether it's *valid*
within our schema.  But it's quite hypothetical; it's possible this
whole concept of "matching" will turn out a lot more complex than that.

There's a tossed out syntax for a schema DSL in a comment.  This is
utterly unscrutinized and should not be taken too seriously yet.

The example code at the bottom declaring some type system is code that
*could* be used, but is mostly for demonstration and early dev
purposes: in the long run, we *do* want to come up with a DSL, and all
the relevant grammers, parsers, and so on for using that as an
implementation-agnostic source of truth.  At that (far future) point,
this kind of code would be used internally to represent what's been
parsed out of the DSL; but users shouldn't really be writing it.
(That's a long-winded way of saying "yes, some parts of that code are
extremely not DRY and would be error prone if written manually"; and
indeed, they would, and thus the point is not to.)
Signed-off-by: Eric Myhre <hash@exultant.us>

Finally got the bind and free impls on the same page.

Surprisingly, the bind node can work without being a ptr itself.
I'm not sure if that'll last, but let's try to roll with it.
If we *can* keep that property, it might reduce GC impact in a pretty
significant way.

Added a 'fluent' package.  It's just a twinkle in my eye so far, but it
might represent the nicest way through all the discussed issues.
Nodes shouldn't have to be panicful; and users shouldn't have to do all
error handling manually either.  A package full of fluent interfaces
that know what's going on seems to be the only way to get both.
But we'll see how this shakes out.  Maybe typeful traversers will
make the whole thing end up more coreward than being relegated to a
helper package.  I have no idea.
Signed-off-by: Eric Myhre <hash@exultant.us>

And add a bunch of type-specific ones for helpfulness.

But immediately re-reviewing this further.  What we've done here is
handle leafs very oddly, because it would seem to avoid unnecessary
wrapping on the leaf nodes.  But the result seems to be much uglier
code than it needs to be, and implies that we've got all sorts of
additional special cases required to either handle or reject binds
of primitive-kind fields.  And that seems... maybe not a great trade.
Signed-off-by: Eric Myhre <hash@exultant.us>

Note: I am not very pleased with how many deps this core and simple
data type has.  These are basically all for the hashing, and very, very
little actual hashing code at that, and I really wish the rule about
"a little vendoring is better than a little dependency" would be
applied here -- pulling in x/crypto and x/sys so that I can use a
type that's *a wrapper around string* is thoroughly ridiculous.
Signed-off-by: Eric Myhre <hash@exultant.us>

Some comments on the interface as well.  Almost certain that traversal
should be pulled off the node interface itself.
Signed-off-by: Eric Myhre <hash@exultant.us>

This has been sitting on my hard drive quite a while already, so I
decided to clean it up juuuust enough that it compiles so I can push
it.  But as you can see, there's a LOT of TODOs dangling here.

I'm not at all still sure those comments about "root node" in the
interface are going to stand up to scrutiny over time.  There's
definitely going to be some distinction between "Node we're traversing"
and "Node we are putting in a serialized block of bytes and can
think it's reasonable to address by hash", but figuring out the best
ergonomics of that is probably going to take a while.

I'm going to start on another simpler just-backed-by-maps-n-stuff Node
impl before going deeper with this one, because I suspect it'll make
tests easier to write, which will then make this whole pile of
reflection easier to test down the road as well.
Signed-off-by: Eric Myhre <hash@exultant.us>

Hopefully if we come up with a good way to flip one node impl into another,
we'll be able to come up with an easy interface for "this is referencing
an in-memory structure type of mine, but hashed as CBOR native standard".

We'll see if this is a good idea or not.  It's either a newer, cleaner,
lessons-learned set of libraries for using IPLD; or, it'll turn into a
"lesson-learned" itself and we'll fold the learnings back into the
existing libraries.  Time will tell!