Rewrote updating RDF literals, supporting rdf:langString and the XML Schema datatypes supported by PGC.

* [RDF_CLEAN,SC] Removed rdf_convert_datatype/6, superseded by rdf_update_literal/7.
* [RDF_DATATYPE,SC] Removed rdf_lexical_map/3, superseded by rdf_literal_map/4.
* [RDF_LITERAL,NEW] Added rdf_literal_map/4 and rdf_update_literal/7.