Vincent Lefèvre reported experiencing an index display performance
issue.  This occurred with messages containing many recipients.  He
had many index color lines containing ~l.  The ~l ended up being run
over and over on these messages, resulting in a noticable slowdown
displaying the index.

This patch adds caching for just a few of the pattern operations (~l,
~u, ~p, ~P) that are potentially expensive and also don't have
arguments.  The caching is only enabled for operations repeatedly
matching against the same message: color, hooks, scoring.

The caching is fairly targeted, but isn't that invasive or
complicated.