add bindings for critical section API

[ngoldbaum]

refs #4265, specifically #4265 (comment), ultimately we want to use this in GILProtected and GILOnceCell.

#4439 is an alternate approach that also uses this API for dict iteration (following the suggestion in the docs for PyDict_Next). I'm holding off on using this API in that spot because of the discussion in that PR.

That PR doesn't use opaque_struct on the GIL-enabled build, but I think that's actually the correct thing to do, since it's an opaque struct in the regular ABI:

https://github.com/python/cpython/blob/556e8556849cb9df0666629b0f564b5dd203344c/Include/cpython/critical_section.h#L70-L71

I'm not exposing the begin/end critical section macros because they include braces and are not expressible in rust.

[bschoenmaeckers]

I'm not exposing the begin/end critical section macros because they include braces and are not expressible in rust.

I kind of like the idea of an inline function that will initialise the CriticalSection object for you before calling PyCriticalSection_Begin.

[davidhewitt]

Thanks, LGTM!

Regarding the macros, usually we don't expose these in pyo3-ffi as we can't match the C semantics. But we can add a safe wrapper in pyo3::sync!