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We add node/tests.SchemaTestAll to simplify this task, meaning we don't
need to duplicate all test func declarations in node/bindnode.
SchemaTestAll is also flexible enough to allow running multiple
sub-tests per schema test in the future.

There were two remaining places in node/tests that still weren't using
ipld.DeepEqual, so fix those.

Finally, bindnode needed a couple of changes to fully support
ipld.DeepEqual. Most notable is iteration over maps, which required a
bit of a refactor to keep ordered keys.

Along with a generic Engine interface, so that they can be reused for
other ipld.Node implementations, such as bindnode.

node/bindnode will start using these in a follow-up commit, since this
one is large enough as is.

Tested that all three forms of testing schema/gen/go still work:

	go test

	CGO_ENABLED=0 go test

	go test -tags=skipgenbehavtests

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.

Buffers are not a good option for tests if the other side expects a
reader. Otherwise, the code being tested could build assumptions around
the reader stream being a single contiguous chunk of bytes, such as:

	_ = r.(*bytes.Buffer).Bytes()

This kind of hack might seem unlikely, but it's an easy mistake to make,
especially with APIs like fmt which automatically call String methods.

With bytes.Reader and strings.Reader, the types are much more
restricted, so the tests need to be more faithful.

This new system focuses on table-driven tests, and leans heavily upon
json as a shorthand for expressing fixtures.

It also makes a great deal more effort to exercise the different
features of nodes (and their paired representation nodes) from all
directions at once for each test datum, rather than requring that all
be written out manually.

The result is that the struct tests we've renovated have a lovely
diffstat shrinkage: 111 insertions, 299 deletions...

And yet the smaller line count results in *more* coverage.

(Okay, the linecount increase for the testcase structure and helper
methods is much bigger than the savings in fixture size... but,
only *so far*.  I assume this will continue to pay off in the future.)

Relatedly: a bug in struct map representations has been fixed.
(It was the sibling of 5f589653, embarassingly.)  Thank goodness we now
get proper coverage of this area.

There's a few TODOs left to further expand the exercises, but those
can slot in easily in subsequent commits.  Same goes for further
expansion of usage of this new system.