Add a simple class to `libs/` for memoizing functions by caching return values
in APC. I decided not to make this an external library just yet because I see
this as potentially a part of a larger functional programming library.
Doesn't use APCBagOStuff for two reasons: (1) avoid dependency on MediaWiki
code; (2) ability to pass third &$success parameter to apc_store, to
distinguish between cache misses and cached false values.
Use this in ResourceLoaderFileModule to cache CSSMin::remap.
Change-Id: I00a47983a2583655d4631ecc9c6ba17597e36b5f
