gh-127295: ctypes: Switch field accessors to fixed-width integers

[encukou]

See the issue.

This refactoring has a miniscule but consistent performance benefit (1.01x geometric mean, no known cases with a slowdown). See my bench script & results. Thanks @vstinner for pyperf and instructions for isolating CPU cores!

For the repetitive parts, this uses a combination of macros and Argument Clinic for code generation (inline, so one can easily inspect the results). This is the most readable/maintainable of various approaches I tried.
(No, I'm not a fan of the giant macros, but it beats both code generated from f-strings that lack syntax highlighting, and external multiple-include files. Oh, and the /////////// lines make it easy to see misplaced backslashes.)

Several related changes are included:

• Pass a meaningful size argument, rather than zero, to non-bitfield accessors. (This would be great to have for size-generic functions, and it should be a private implementation detail. I want to do this now and get the asserts in the wild, to see if I missed someone who's relying on the detail.)
• Use a switch in _ctypes_get_fielddesc, rather than a linear search. (This requires that there are now several boring chunks with a line for each of the format codes, sbBcdCEFgfhHiIlLqQPzuUZXvO, but Argument Clinic makes this bearable.)
• Rearrange struct fielddesc to move all the accessors together, making code generation a bit easier
• Motley consistency improvements, like changing BSTR in function names to its code char X, to match the other accessors

• Issue: ctypes: Switch field accessors to fixed-width integers #127295

[encukou]

(Yes, this is wrong as-is; hopefully due to a stupid mistake on my part.)