Score flex-style completions according to match tightness

Score flex-style completions according to match tightness

The new completion style needs to score completion matches so that we
can use it later on when sorting the completions.  This is because
"foo" can flex-match "foobar", "frodo" and "barfromsober" but we
probably want "foobar" to appear at the top of the completion list.

This change introduces a scoring formula and adds scoring hints in the
candidate string's `completion-score' property.

* lisp/minibuffer.el (completion-pcm--hilit-commonality): Propertize
completion with 'completion-score
(flex-score-falloff): New variable.