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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.

Lots of TODOs and polishing to do, but it passes the schema/gen/go tests
with minimal changes.

Follow-up commits will continue filling in the gaps and adding
documentation, notably examples. We'll also make the tests run by
default, after a bit of refactoring.