Cyclic dependency between Concepts and Semantics

[pchampin]

I noticed here (and James actually noticed that before me) that RDF 1.2 Concepts normatively depends on RDF 1.2 Semantics, which in turn depends on RDF 1.2 Concepts.

I believe that Concepts should not normatively depend on Semantics, I'll try to figure out where this comes from.

[pchampin]

Found it, it is in Annex B about skolemization. Will make a PR.

[ktk]

PR: #251