Big cleanup of RDF(S) axioms and rules, materialization.

* [RDF_BNODE_MAP,SC] Renamed predicates, variables, documentation, simplifications.
* [RDF_ENT,RDFS_ENT,SC] Axioms, explanations, rules are now multifile and shared between modules, i.e. prefixed by `rdf:`.
* [RDF_MAT,SC] Removed start_materializer/3, sanitized option definitions, increased modularity in materialize/2 predicate.
* [RDF_ENT,RDFS_EXT,FIX] Enumeration of iterator predicates was fixed.
* [RDF_MAT,SC] Various cleanups.
Working on backward reasoning for RDF.