mypy

0ba1d99e - [mypyc] Implement serialization and deserialization of mypyc IR (#7734)

[mypyc] Implement serialization and deserialization of mypyc IR (#7734) This is work towards incremental compilation (mypyc/mypyc#682). Serialization format is JSON blobs similar to mypy itself. The serialization and deserialization is not actually connected to anything yet, except that run tests now validate that the serialization round trips.