Finalized the conversion of modules ASCII and PARSE_EXT to modules DCG_GENERIC and DCG_ASCII.

* [ASCII,SC] Removed this module. Functionality is duplicated by module DCG_ASCII and DCG_CARDINAL.
* [C,SC] Removed this module. Functionality is duplicated by module DCG_C.
* [CSS,SC] Rewrote code using DCGs.
* [DCG_ASCII,EXT] Added alpha_numeric//0, ascii//0, graph//0.
* [DCG_ASCII,SC] Renamed print2//0 and tab//2, which conflicted with builtin predicates.
* [DCG_ASCII,SC] Relocated exponent_sign//0 to module DCG_CARDINAL.
* [DCG_C,EXT] Finalized the DCG version of C-compliant string conversion.
* [DCG_CARDINAL,SC] Added exponent_sign//0 from module DCG_ASCII.
* [DCG_GENERIC,NEW] Added dcg_multi//2 for multiple consecutive occurrences of a DCG rule.
* [DCG_GENERIC,NEW[ Added dcg_questionmark//1, same behavior as =|(DCGBody ; "")|=.
* [DCG_HTML,NEW] New module for converting HTML-compliant strings.
* [DCG_YEAR,SC] Use of dcg_multi//2 (see module DCG_GENERIC) simplifies year_uncertainty//1.
* [DCG_YEAR,SC] Relocated convert_epoch/3 and epoch//1 from module PARSE_EXT.
* [HTML,SC] Relocated DCG-based code to DCG_HTML.
* [LIST_EXT,NEW] Added member_default/3.
* [META_EXT,NEW] Added switch/[2,3] more similar to the control structure from procedural languages with the same name.
* [META_EXT,NEW] Renamed and relocated old switch/3 predicate to module LIST_EXT.
* [PARSE_EXT,SC] Removed this module. Functionality replicated in modules DCG_CARDINAL and DCG_GENERIC.
* [RFC_3066,SC] Rewrote code using DCGs.
* [SVG,SC] Rewrote code using DCGs.
Cleanup of the various DCG modules.