Improved the patch further on feedback from Stefan.  It now also uses
completion-lazy-hilit, which further improves performance.

I'm not sure if completion-lazy-hilit-fn needs to be saved and restored
(like group-fun is being saved and restored).  The current version of
this patch doesn't save completion-lazy-hilit-fn, and that seems to work
fine, because completion-lazy-hilit-fn hasn't changed by the time we
call completion--lazy-insert-strings.  But maybe this is not robust in
some case?