Fixit is incredibly slow with many files.

Related to #457 I was doing some profiling on our custom fixit rules, and determined that the number of files being linted was the primary determinate in fixit runtime.

To that end, I've setup a repository that demonstrates the performance issues. It contains a large number of empty `__init__.py` files in a tree of modules, about 41,000 files in total. It has a single fixit rule with a `visit_Module` that does nothing except return. Running fixit against this tree pegs all of my cores to 100% and takes about 40 seconds to complete.

https://github.com/jarshwah/experiments-fixit is the repository which I'm using to document my findings. So far I've only looked at the file-finding, but plan to dig in further where time permits.