e73b0d6: Implement unprettification of symbol at point

[Mark Oteiza]

>    * lisp/progmodes/prog-mode.el: Implement feature for unprettifying the
>    symbol at point.
>    (prettify-symbols--current-symbol-bounds): New variable.
>    (prettify-symbols--post-command-hook): New function.
>    (prettify-symbols-unprettify-at-point): New defcustom.
>    (prettify-symbols-mode): Use it.
>    (prettify-symbols--compose-symbol): Use them.

All the new changing of text properties is modifying the buffer.

Also, why is this defaulting to on?