Working on automated translation from ABNF to DCG.

* [ABNF,NEW] Added wrappers 'BIT'//1, 'DIGIT'//1, and 'HEXDIG'//1.
* [ABNF2DCG,NEW] New module for automatically translating ABNF grammars to DCG grammars. TBD.
* [DCG,EXT] Rewrote dcg//[1,2,3,4] to dcg_nonvar//[2,3,4] and dcg_var//[2,3,4]. Candidate for replacing dcg_multi//.
* [DCG_CONTENT,NEW] Added dcg_code//1.
* [DCG_CONTENT,EXT] dcg_cistring//1 no longer uses SWI-Prolog strings but code lists.
* [RDF_META_AUTO_EXPANCE,DOC] Minor improvements to documentation.
* [RECORD_JAR,FIX] Fix in DCG rule for encoding.
* [FIX] Several missing module imports.