Fix FileCache

[scratchyone]

As reported in #40, FileCache isn't usable, as it only implements Cache<Path>. However, as Path is unsized, it's impossible to actually pass in a Path, making FileCache unusable. This PR just changes the implementation to use &Path instead. In my quick testing, it appears to now work without any issues.

[nevakrien]

How about we dont break backwards compatibility by simply adding ANOTHER implementation of Cache as opposed to changing it.

So file cache would work with both Path and &Path and we can just merge it right now