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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.

Int is now basically all common stuff.  Now I'm really gonna quit.

Interestingly, the total line count isn't going down very fast.
108 insertions, 165 deletions, according to the git diff stat.
Evidentally the overhead of things opting into this is darn near
the amount of lines saved in many cases.

This one turns out to be different betweens scalars and recursives,
if in a small (and fortunately consistent) way.

These are getting increasingly straightforward as most of the hard
problems have been ironed out already when getting the other recursives
(maps and structs) sorted.

Now, we just need to stamp out the rest of the scalars, and I think
this codegen stuff is at least early-alpha level usable!

Also in this commit: added some new error types and fixed up the
basicnode.List implementation to use it, removing some todos there.