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Extracted a few last comment hunks that were useful and that's
the end of it.

The migrated tests only mostly pass (but, remember, we've been several
commits in a row already where we're making checkpoints to keep this
refactor managable, and CI is not full green across the board for
some of them).  I've updated some of the error text assertions,
but there's other breakage as well.  One is just other todos.
Another is... It may be time to switch test libraries.  What go-wish
(and transitively, go-cmp) is doing with `[]error` values is...
not helpful.

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.

Since https://github.com/ipld/go-ipld-prime/pull/121, presence of
fields is actually checked... but that code also doesn't understand
implicit fields yet, which makes us need a lot of filler.

Also the lack of the "members" field for unions?  That was just plain
wrong.  Good think we're catching things like that now.

draft of schema types using codegen for data model, with a package for the fully validated data which is implemented by retaining and accessing into the raw data.