Expose unprefixed variants of `_PyCFunctionFast` and `_PyCFunctionFastWithKeywords`

[davidhewitt]

METH_FASTCALL is documented as part of the stable API since Python 3.10.

The accompanying function pointer typedefs _PyCFunctionFast and _PyCFunctionFastWithKeywords have leading underscore names, which I understand to hint at these being private / internal APIs.

I think this is potentially an oversight and these function pointer typedefs should have new public names PyCFunctionFast and PyCFunctionFastWithKeywords?

Linked PRs

• gh-114626: add PyCFunctionFast and PyCFunctionFastWithKeywords #114627
• gh-114626: Add again _PyCFunctionFastWithKeywords name #115561

[vstinner]

Thanks for making these two names public @davidhewitt!