Several smaller updates to RDF_BUILD and type checking.

* [META_EXT,NEW] Added unless/2 syntactic sugar predicate.
* [RDF_BUILD,SC] Remove rdf_update_datatype/5. Functionality incorporated into rdf_overwrite_datatype/5.
* [RDF_BUILD,EXT] Behavior change for literal assertions without a language tag: assert as literal/1, not literal/2.
* [RDF_BUILD,EXT] Added more sophisticated type checking for rdf_overwrite_datatype/5.
* [RDF_TYPECHECK,NEW] New module with simple predicate for checking RDF term types.
* [TYPECHECK,SC] Renamed module and main predicate.
* [RDF_BUILD,FIX] rdf_overwrite_datatype/5 should succeed if the new value is already asserted.