* lisp/hi-lock.el: Rework the default face and the serialize regexp code.

* lisp/hi-lock.el: Rework the default face and the serialize regexp code.
(hi-lock--auto-select-face-defaults): Remove.
(hi-lock-string-serialize-serial): Remove.
(hi-lock--hashcons-hash): Rename from hi-lock-string-serialize-hash;
make weak.
(hi-lock--hashcons): Rename from hi-lock-string-serialize, return an
equal string.
(hi-lock-set-pattern): Adjust accordingly.
(hi-lock--regexps-at-point): Simplify accordingly.
(hi-lock--auto-select-face-defaults): Remove.
(hi-lock--last-face): New var to replace it.
(hi-lock-read-face-name): Rewrite.
(hi-lock-unface-buffer): Arrange for the face to be the next default.