RFC5646 now seems to use the IANA RDF registry for subtag checking.

* [DCG_CONTENT,NEW] Added dcg_codes//1 processing a codes list 'as is'.
* [DCG_GENERIC,FIX] Rewrote dcg_separated_list//2.
* [DCG_WRAP,FIX] Rewrote dcg_word_wrap//2 to fix several open-standing problems:
** Lines were not filled with spaces until the end.
** Word may have the exact same length as the space available in the current line.
** Would not process words that are longer than the maximum line length at all.
* [DEB_EXT,EXT] printw/1 now prints code lists as atoms.
* [GRAPH_TRAV,FIX] Solution was not returned in one choice branch. Found thanks to the new singleton discover in swipl!
* [RDF_GRAPH,EXT] rdf_graph_source_file/2 is now (?,?), i.o. (+,-).
* [RFC5646,EXT] Load the IANA registry, RDF version, to perform the class checks for language (sub)tags.
* [RFC5646_IANA,FIX] Fix for namespaced argument in rfc5646_class/2.
* [RFC5646_IANA,EXT] If the RDF graph is already loaded then this does not have to be done again.
* [XSD,FIX] Several fixes due to spotting singletons according to the new swipl checks.