Alternatives to using private API for conversion to/from CPython int's

[skirpichev]

Currently, we are using a bunch of private CPython functions to provide fast mpz <-> int conversion: see GMPy_MPZ_From_PyLong, GMPy_PyLong_From_MPZ and https://github.com/aleaxit/gmpy/blob/141eb88c9c4ab810d7988f0ebd17c83a6c11a8b7/src/gmpy2_convert.h#L135-L155 There should be a better way!

I see several variants to deal with this problem.

1. The CPython 3.13 got PyLong_AsNativeBytes() and PyLong_FromNativeBytes() function to import/export arbitrary int's. We can use these functions and handle small integers case separately with PyLong_AsLong()/PyLong_FromLong(). In general, this should be much slower than the current solution. On another hand, I think that for real world scenarios - small integers case should be most important.

2. There is a proposal to add mpz_import/export-like functions for PyLong's C API, but it seems that CPython core developers aren't very enthusiastic on this idea (see https://github.com/python/cpython/issues/111140#issuecomment-1928654497).

3. Another possibility is the mpz_limbs_write()-like API to access unsigned bigint as an array of "digits", see this. With that kind of API on the CPython side - the gmpy2 could do fast conversion to/from int's using mpz_import/export functions.

I'm planning to open a discussion thread on the d.p.o, that will propose the 3rd option, but first I would appreciate any feedback here. CC @oscarbenjamin: perhaps this does make sense for the python-flint.

[skirpichev]

See https://github.com/capi-workgroup/decisions/issues/31