Cache results to prevent linting unchanged files

[amyreese]

[thatch]

Discussed at pycon:

• would be great if trailrunner did this, so ufmt/usort also got the benefit
• pluggable local (either file or sqlite) and memcache
• how do you invalidate (file hash isn't enough, you also need need fixit ver and rules, and I haven't considered full repo metadata yet)