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Including LookupBySegment and LookupByNode for both maps and lists, but
also As<T> for the representation form of all scalar kinds.

While at it, expand the interface implementation assertions, too.

Note that we had to make the node/basic tests external,
for the sake of testing LookupByNode without inserting an import cycle.

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