FFI module cleanup

The modules in src/ffi have become a bit out-of-date, and also quite disorganised so it's kinda hard to figure out what's missing.

Since Python 3.9 the cpython include files have been split so that (roughly) the limited api is in cpython/Include/ and the unlimited api is in cpython/Include/cpython/. I propose for simplicity in maintaining the pyo3::ffi module we should reorganise code to match that structure.

I suggest that we do the following things:

• reorganise ffi/mod.rs to be alphabetical and have // skipped foo.h comments for each header present in cpython master which is not present in our bindings at the moment.
• add ffi/cpython/mod.rs to contain the unlimited api headers, and do the same as with ffi/mod.rs.
• for each file in ffi/, reorganise the definitions in it appropriately into ffi and ffi/cpython parts which match the cpython master definitions. This should include:
  • Put the existing definitions in the same order as upstream cpython.
  • Either:
    • Add // skipped foo for each definition foo we don't currently have in our definitions.
    • Add new definition for each we don't currently have in our definitions.

The idea with each // skipped foo being in alphabetical order is that it's then hopefully easier for us to compare against upstream in the future.

With the new cpython folder for the unlimited api, it might be reduce a lot of #[cfg(Py_LIMITED_API)] chatter as we'll only need that once on the mod cpython definition.

Also, it could be interesting to not have pub use self::cpython::* in ffi/mod.rs, so that users are forced to use unlimited api symbols as ffi::cpython::foo. But that's probably quite breaking, so I can go either way on it.

Perhaps in general the above guidelines of how to organise our ffi modules should go in CONTRIBUTING.md ?

Start with src/ffi/mod.rs and src/ffi/cpython/mod.rs, then compare each file under src/ffi with the corresponding CPython master headers. Reorganize definitions into ffi and ffi/cpython in upstream order, adding the requested skipped comments or missing definitions; the work is done when the modules consistently match the upstream split.