Maintain a cache of GitHub API responses

[woodruffw]

This shouldn't necessarily be cached at the API response layer itself, but we should cache in in some form. In particular:

• A symbolic ref cache for each user/org slug requested (so we don't hammer the same APIs over and over)

[woodruffw]

#7 does this, but not to my satisfaction: the caches for these things should be present at the "run" layer, so that individual audit can reuse them. As-is, each audit needs to maintain its own cache.

[woodruffw]

https://github.com/moka-rs/moka looks like the right fit in terms of a robust cross-context caching library.

[woodruffw]

Done with #38.