Introduce text-coverup API.

Introduce text-coverup API.

* lisp/progmodes/prog-mode.el (text-coverup-alist): New variable
supporting regular expression text coverup entries.
(text-coverup-default-compose-p): Add default compose predicate
paralleling prettify-symbols-default-compose-p.
(text-coverup-compose-predicate): Add buffer-local variable for
user-defined composition predicates.
(text-coverup-uncover-at-point): New defcustom.
(text-coverup-add-coverup-entry)
(text-coverup-add-coverup)
(text-coverup-remove-coverup)
(text-coverup-remove-coverups)
(text-coverup-remove-all-coverups): Add text-coverup API
functions.
(turn-off-text-coverup-highlighting)
(turn-on-text-coverup-highlighting): Add top level text-coverup
management functions.