GitLab

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