Parse files in parallel when possible

[ilevkivskyi]

The idea is simple: new parser doesn't need the GIL, so we can parse files in parallel. Not sure why, but the most I see is ~4-5x speed-up with 8 threads, if I add more threads, it doesn't get visibly faster (I have 16 physical cores).

Some notes on implementation:

• I use stdlib ThreadPoolExecutor, it seems to work OK.
• I refactored parse_file() a bit, so that we can parallelize (mostly) just the actual parsing. I see measurable degradation if I try to parallelize all of parse_file().
• I do not use psutil because it is an optional dependency. We may want to actually make it a required dependency at some point.
• It looks like there is a weird mypyc bug, that causes ast_serialize to be None sometimes in some threads. I simply add an ugly workaround for now.
• I only implement parallelization in the coordinator process. The workers counterpart can be done after Split type-checking into interface and implementation in parallel workers #21119 is merged (it will be trivial).

cc @JukkaL

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅