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add an optional reifier into the link system process. The reason to do this is to capture the link
system itself when reification happens, in case it needs to be put into the node, which is often not
accessible by the time you have a node. another alternative would be to make this specific to
selector traversal, similar to LinkNodePrototypeChooser

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.

Fixed a symbol to be exported that's needed for this to be possible
when outside the package.  This still probably deserves an interface,
too, though.  Comments on that also updated, but we'll still leave
that for future work (more examples of more ADLs wanted before we try
to solidify on something there).