Allow completion frontends to highlight completion strings just in time

Allow completion frontends to highlight completion strings just in time

This allows completion-pcm--hilit-commonality to be sped up
substantially.

Introduce a new variable completion-lazy-hilit that allows for
completion frontends to opt-in an time-saving optimization by some
completions styles, such as the 'flex' and 'pcm' styles.

The variable must be set by the frontend to a unique value around a
completion attempt/session.  See completion-lazy-hilit docstring for
more info.

* lisp/icomplete.el (icomplete-minibuffer-setup): Set completion-lazy-hilit.
(icomplete--render-vertical): Call completion-lazy-hilit.
(icomplete-completions): Call completion-lazy-hilit.

* lisp/minibuffer.el (completion-lazy-hilit): New variable.
(completion-lazy-hilit): New function.
(completion-pcm--hilit-commonality): Use completion-lazy-hilit.