Add iterators and loop paths for codepoints and glyphs

 * src/code/string.lisp:
   * Add WITH-STRING-CODEPOINT-ITERATOR to allow iterating over the
     codepoints in a string.
   * Add WITH-STRING-GLYPO-ITERATOR to allow iterating over the
     glpyhs in a string.
 * src/code/exports.lisp:
   * Export WITH-STRING-CODEPOINT-ITERATOR and
     WITH-STRING-GLYPH-ITERATOR. 
 * src/code/loop.lisp:
   * Support (loop for cp being the codepoint of string ...) which
     extracts the consecutive codepoints from the string. Allow
     codepoints, code-point, and code-points as aliases of codepoint.
   * Support (loop for g-string being the glyph of string ...) which
     extracts each glyph (as a string) from the string.  Allow glpyhs
     as an alias.
 * tests/extended-loop.lisp:
   * New file of tests for the new loop paths.
 * src/i18n/locale/cmucl.pot:
   * Update.