Address code duplication in mypyc/lib-rt

[ilevkivskyi]

There is a bunch of code duplication in mypyc/lib-rt, but it looks like there is no way to resolve this in a reasonable way. So instead I factor out duplicated code into a separate module, and add a test to verify files are synced.

Note I type-check lib-rt separately now. Whatever I tried, I am getting weird error either like

Cannot find implementation or library stub for module named "build_setup"

or

Source file found twice under different module names: "mypyc.lib-rt.build_setup" and "build_setup"

etc. This is kind of a known problem, that we need to address long-term.

cc @JukkaL

[github-actions]

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