Add a pattern_cache_t to speed up a few repeated matches.
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.
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