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Codegen: approaching self-host

This thing need to be replaced with non-placeholder code soon; it's
getting pretty nasty.

This is full of mundane plonking away at adding stars and ampersands,
of which approximately none are interesting.

(I'm particularly frustrated by this because these are all placeholder
types, and we're getting *very* close to replacing them, as we get
closer and closer to self-hosting... at which point all of this bonking
about will be made totally irrelevant.  And yet to close the last mile,
these "small" fixes are surprisingly irritating.  Ah well.)

The bits that *are* interesting:

- the Spawn functions for type info now take strings rather than types
(so that they don't provoke a cycle problem for the user when
constructing the information to describe cyclic type info);

- all of the Type info structure hold a pointer to the TypeSystem, and
use that to look up reified Type info for related types, so that their
methods don't force the caller to do that themselves.

(The TypeSystem pointer was already there, amusingly; just never before
initialized, because it hadn't turned out to be load-bearing yet.)

It also would've been possible to just change all the methods on the
Type types to return TypeName rather than full Type info.  That would
avoid the need to use a TypeSystem pointer.  I didn't because:

Overall, this was done in such a way as to minimize the diff that
impacts within the templates.  This was a goal because updating
templates is a fair bit more work than other code due to the weak
compiler support.  And we'll end up reviewing and changing these
methods when we get to our goal of self-hosting generation of the
schema types from the schema-schema, so, it's not worth pushing around
diffs in that same area when they'd be sure to be churned under soon.

These were apparently never used, or their users got churned under
at some point and I failed to clean these up at the same time.

It's a good idea to have tests that are agnostic to codegen vs
runtime type semantics implementations... but at this point we should
reengage with that by just extracting the stuff currently in the
codegen package's tests, because it's much more comprehensive and
also almost entirely abstracted already (it just happens to be in the
wrong place in the file/package hierarchy for no particular reason).

Codegen of unions, and their keyed representations

This is a clear and simple solution to the problem.

(I think I may have gotten so optimistic about type systems for a while
that I forgot that returning nils is an option.  mmmm.)

The docs on NodeBuilder already even nodded towards this possibility,
so I've just further clarified that.

After staring at this method for a while, I begin to wonder if it even
is all that useful.  To use it sensibly on structs or unions (for any
purpose other than checking if something is *not* a field/member),
you have to figure out which keys you can ask it about... which...
means you'd need the type info, to be able to enumerate that!  At which
point you'd just be able to look at the type info, and wouldn't really
need to ask the builder about ValuePrototype in this way at all,
rendering the whole thing moot.

But removing it seems a little drastic, too.  And would leave questions
about how to do those inspections on untyped things.  So.  Despite the
nibbles of oddity around this, perhaps this is still the least-bad
design that can make these situations legible.

I can't quite claim tests passed on the *first* shot... but,
the first shot after mostly syntactical (rather than semantic) fixes?
Yeah, actually.  That was pretty fun.

Snuck in a bit of DRY'ing up.  The repetition in BeginMap methods
got to me, and was low hanging fruit, so I extract that from unions
and also backported it to structs.

Errors got some work in this commit, because it turns out I've
straightjacketed myself by not allowing "fmt.Errorf" due to imports.
There's a lot to do there, and I only tackled what was directly
critical to get this commit about unions across the finish line,
but there's a few remarks in comments about where to go next.

Some more comments about future work on the type info holder types
also appears; I'm starting to skid on those placeholders and their
issues more and more.  I really hope we can get to replace those
sooner than later.

And... also, yes, the idea of not having a "focus" state field in
assemblers really bit it, hard, as speculated in the previous
commit message.  I ended up using 'ca' in more switches than I
expected, simply because it's easier to use that than have the
conditonal templating branches that would be necessary to use the other
tagging mechanisms that do also have sufficient info.

One big fixme in the core interfaces for nodebuilders (wince):
the ValuePrototype method can error sometimes, and that hasn't been
accounted for.  Need to make a decision about what to do there.
It's not really an exercised path in practice, but it shouldn't
contain caltrops, regardless of how frequently used it is.
(This probably would've come up earlier, except there's a bunch of
stubs about ValuePrototype in other parts of codegen already; all of
them need backfill, but haven't yet made it to top of the todo heap.)

But despite all the fixmes accumulated, this does bring unions
to be a usable thing, and definitively proves out that the design
still works, even for what turns out to be one of the most complicated
parts of the schema system!  It's very, *very* exciting to add the
checkmarks to this part of the feature table -- it's one of the places
I most feared "unknown unknowns", now it's put to bed.  Woot!

... for the type-level assemblers, anyway.

We *still* can't do e2e tests on this that include compiling the
generated code; not until we get at least one representation strategy
implemented.  All the templates exercise, though (even though you
might have to uncomment some of the test "skip" lines to do it).

I'll move on to doing the keyed representation next, and that will
be the thing we use to prove out all of this stuff.  It should be
the easiest one to do now, too, since it's semantically very very
similar to how we made the type-level behaviors work.

These are turning out to be *very* fun.

In the process of detailing how child assemblers work for unions,
it has become time to consider how separately-allocated child
assemblers are handled, and what their existence implies about
the rest of our logical rules how all the assemblers behave in concert.
Like so many things in this endeavor... the far-reachingness of
the implications of even seemingly very mild choices is... high.

I think it's quite likely that the current claims about union
assemblers not needing extra state for set/focus might turn out to be
redacted in upcoming commits, and for the strangest reason: we might
actually need that state to be persisted (outside of 'w') so that
the *reset* mechanism, of all things, can be efficient.
We'll see; still sort of tumbling that thought around in my mind.
There are various options.

Outline for keyed representation, but only enough to get some more
things compilable and start getting all the templates exercised.

Still can't really e2e test the things we've got without getting
the rest of it and its builders finished.  It's coming along pretty
reasonably, though, it seems.

Generates the type structure itself, and a marker interface.
Not much else yet.

Wired test harnesses and plugged out one example.

It's a bummer we don't really have a great way to poke part of it into
running until the whole thing satisfies the generator interface.
But it'll come soon enough.

Golang templates are mostly pretty smooth to get started with,
but this particular issue is really a bizarre tripwire, imo.

Will clean up the existing "range modifies dot unhelpfully" messes
sometime over the rainbow; I just refuse to make more of them today.

Also make some of the placeholder construction functions.
(Not all of them.  Just the ones I intend to use first.)

There was some very unfinished placeholder stuff going on there.
(And ironically, it wasn't written in a very clear union-like way;
it gets a lot less icky when it's rewritten so that it's uniony.)

The appearance of the word "any" there has started to perturb me;
"any" is a concept that we also need to describe in schemas, and that
type has nothing to do with it.  It's more of a base mixin.

It's kind of dark.  I've been leaving remarks strewn around github
for a while now to the general effect of "please please please
prefer keyed unions whenever you can; the others can be Not Fast".
But it's even more nasty in terms of details like error handling
than I think I had yet realized.  Sigh.

Oh well.  Remember: we exist to describe data that people *have made*,
not just the data that we want to encourage them to make in the future.

Expect this to evolve slowly over and the course of *many* commits.
Unions are exceptionally tricky, and their various representations
are by far the most complicated parts of IPLD Schemas.

In particular, the fact that how to interpret data *inside* a union
may vary, whilst some representations may not actually tell us which
union member we have until later in the stream... is going to produce
a lot of absolutely fabulous complexity.  Look forward to it.

API updates for v0.5: the renamening

codecov, I need to you understand that I don't value your input, you're not even enabled on this repo, and you're especially not welcome to continue marking my builds as "failed" by your standards that I, again, do not value

I'm embarassed that dbab5316 wasn't sufficiently complete in removing this thing; I hope this now is the end of it.

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.

... and it will further become LookupByNode shortly, but that will be
a separate commit.

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

Codegen update -- Assemblers, and many new representations

(It's a small diff -- just reflecting the fix for map returing absent.)

... almost.  In attempting to put the sections about absent value
handling into place, I realized there's a decently sizable bug there
already.  I'm going to document this for now, move on, and come back
to it later... so it will continue to take up space in a "wip" doc.
The description of the situation is at least cleaned up now.

I spent some time poking around at other applications that I'd want to
be able to use some of this stuff on, and the lines about "minimalist"
come from that.  I suspect that in ideal outcomes, I'd like to be able
to sometimes be able to generate the type structures and native/typed
APIs for a Schema definition... but perhaps not generate all the stuff
necessary for the interface monomorphization, because perhaps I just
want an enum purely for internal program purposes; surely we should be
able to just do that.  We'll see though.  Maybe that's scope creep
as much as anything else.

All "genparts*" files are reusable bits of template;
all other "gen[A-Z](.*)" files are per kind.