* lisp/character-fold.el: Code simplifications

* lisp/character-fold.el: Code simplifications

(character-fold-table): Reduce the scope of a variable.
(character-fold-to-regexp): Change logic to work directly on the
input string.  It's a little easier to understand, probably
faster, and sets us up for implementing multi-char matches.

* test/automated/character-fold-tests.el
(character-fold--test-fold-to-regexp): New test.