"RDF term" definition should be exported

[pchampin]

Reviewing w3c/rdf-semantics#74, it occurred to me that the defined term "RDF term" is not accessible from outside this document, which is a problem (RDF semantics, and probably other specs, need to reference it).

There are probably other definitions in this spec that need to be exported. I'll try to make a list of them, and a PR.