Suggestion: use Piscina for faster codemods

[NickHeiner]

I've done testing, and found that Piscina is 30-50x faster than in-process transforming. My codebase is 15k+ files, so the speedup really helps.

[eventualbuddha]

This is a good suggestion. I've been meaning to do something like this for a while, just haven't gotten around to it. I'd be happy to review a PR and/or give some guidance on getting started.