Consider generic mechanism for caching results by file/hash

[amyreese]

Caching can help usort/ufmt/etc do less work when formatting larger projects, and this might be the right place to implement a simple caching mechanism.

See omnilib/ufmt#198 for experiments in caching above trailrunner, but could be nice to provide a generic hash-based caching system, with the option for providing an alternative implementation.