Allow the `f_func` field of the `_PyInterpreterFrame` struct to be any object (and rename it)

[markshannon]

In order to better support shim frames, trampolines, Cython frames and other such bits and pieces, we need to relax the requirement that the f_func field is a function.
The only reason for it to be a function is that it needs to hold a strong reference to builtins and globals, as the frame only has a borrowed reference to those. Any object with those references will do.
In fact, for code that has no builtins or globals (many shims and trampolines) there is no need for those references and f_func could even be None.

[iritkatriel]

Does it really need to allow any object? Could you instead define a small number of object types that it can be?

[markshannon]

I don't see any advantage to restricting the possible types. Allowing any object is no worse for any code reading from f_funcobj and more flexible for anything storing to it.

Given the values of f_globals and f_builtins can no longer be trusted, I think I'll make them void * just to make it clear that if you didn't set them, then you can't trust them.

[carljm]

Currently whatever is in f_funcobj also needs to have a func_closure. #101310 demonstrates one way to remove this requirement.

[carljm]

Should this issue be closed, given the attached PR was merged? Or is there more planned here?

[markshannon]

Superseded by #100987