Add librt functionality for lazy deserialization

[ilevkivskyi]

I am going to add actual lazy deserialization in a separate PR, probably next week. It will require one more preparatory PR setting up global state for lookup_fully_qualified() that I will probably add on the weekend.

cc @JukkaL

[ilevkivskyi]

Hm, IIUC I should bump LIBRT_INTERNAL_API_VERSION but then this creates a chicken and egg problem in tests. So that I can't really merge this until I publish new librt version on PyPI.

I guess we should use the local version of librt during self-compilation in tests.

[github-actions]

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

[ilevkivskyi]

This also makes me think that if anyone is using compiled mypy form master, it will break as soon as I merge this. So maybe a better approach is to actually release this in librt first.