[Python-3000] C API for ints and strings

["Martin v. Löwis"]

>> Why is _PyLong_FitsInLong private?
> 
> I don't know; perhaps because it doesn't always give the best answer.

Its sole purpose is to support PyInt_CheckExact. There is some code
that relies that after PyInt_CheckExact succeeds, it is safe to do
PyInt_AsLong. When I defined PyInt_CheckExact to PyLong_CheckExact,
such code would break. Adding this "conservative" estimate allowed
that code to work when the macro was true. As this occurs in some
time-critical places, I did not want to waste time with computing a
correct result.

Regards,
Martin