Use lexical-binding in hilit-chg.el

Use lexical-binding in hilit-chg.el

* lisp/hilit-chg.el: Use lexical-binding.  Remove redundant :group
args.  Doc and formatting fixes.
(highlight-changes-mode, highlight-changes-visible-mode):
Use keyword arguments.
(hilit-chg-display-changes, highlight-changes-rotate-faces):
Quote function symbols as such.
(hilit-x, hilit-y, hilit-e): Define variables on top-level to
silence the byte-compiler.
(highlight-markup-buffers): Improve message format.