Cleanup of code extensions.

* [CODES_EXT,SC] Cleanup of this module, removing several unused predicates (e.g. replacing inside codes lists and splitting codes lists is now done with DCGs).
* [CODES_EXT,DOC] Documentation header expanding on how to replace , split, and strip in codes lists.
* [CODES_EXT,NEW] Added strip_codes[_begin,_end]/3 to be in line with the predicates for atoms (module ATOM_EXT).