* [XML,FIX] Module imports and meta-predicates.
1. Improvements to DCG extensions: dcg_multi// and dcg_until//.
2. Implementation of RFC 5646 standard.

* [DCG_GENERIC,SC] Simplified API: dcg_until_atom//, dcg_without//, and dcg_without_atom// were removed. Functionality was merged into dcg_until//.
* [DCG_GENERIC,SC] Simplified API: dcg_plus//, dcg_questionmark//, and dcg_star// were removed. Use dcg_multi// instead.
* [DCG_GENERIC,EXT] Added options to dcg_until// for choosing between in- and exclusive mode and for letting `Value` be either an atom or a list of codes.
* [DCG_GENERIC,SC] dcg_multi// now takes=|`between(?Min:integer,?Max:integer)|= as an argument for parameter `Occurrences`, instead of two separate arguments.
* [DCG_GENERIC,EXT] dcg_multi// is now greedy, startign with `Max` occurrences and then going down towards `Min`.
* [DCG_GENERIC,EXT] Either the lower or the upper bound is allowed to be unset (i.e., an uninstantiated variable).
* [DCG_GENERIC,EXT] For dcg_multi// a variant that injects a single argument was included.
* [MATH_EXT,NEW] Added rbetween/3 that returns integers from big to small.
* [RFC5646,NEW] Implemented this RFC standard: DCG rules for *well-formedness*, search predicates into the IANA registry will support the future addition of *validity*.
* [RFC5646,EXT] Added IANA registry file language subtag lookup.