Reimplementing datatypes: string, decimal, boolean.

* [DCG_CARDINAL,NEW] Added int_codes//1 from the swipl module dcg/basics.
* [DCG_CARDINAL,SC] Replaced sign//[0,1] with sign//[1,2].
* [RADIX,SC] Removed superfluous predicate number_to_digits/2.
* [RDF_BUILD,EXT] Rewrote rdf_assert_datatype/5 and rdf_retractall_datatype/5 to work with the new XSD implementation.
* [RDF_DATATYPE,SC] Removed this module. All datatypes will be defined within the XSD framework from now on.
* [RDF_READ,EXT] Rewrote rdf_datatype/5 and rdf_has_datatype/4 to work with the new XSD implementation.
* [SC] Renamed relation RELATION to FUNCTION (a specific kind of relation).
* [SC] Separated the codebase for XML Schema 2: Datatypes into various modules:
** XSD for the generics.
** XSD_BOOLEAN, XSD_DECIMAL, SXD_STRING, modules for each primitive datatype.
* [XML_DATATYPES,NEW] Added xml_chars//1.
* [XML_SCHEMA_DATATYPES,SC] Removed most of this module. To be phased out soon.