1. Big rewrite of loading/saving RDF graphs.

* [RDF_SERIAL,EXT] Big rewrite of rdf_load2/2 and rdf_save2/2. Cleaning a lot of complicated predicates, keeping the good stuff.
* [RDF_SERIAL,EXT] Rewrote the old turtle2xml conversion so that it now does any2any conversion (using the new rdf_save2/2 and rdf_load2/2).