1. ADDED: rdf_load_any/2 based the now removed module rdf_serial
          and the new open_any/2.
2. ADDED: rdf_save_any/2 based the now removed module rdf_serial
          and the new open_any/2.
3. IMPROVED: Assert the rdf_format type.
4. CLEANUP: Simplified rdf_serialization/4, removing the unused
            file format argument.