bpo-35134: Add Include/cpython/longobject.h (GH-29044)

Move Include/longobject.h non-limited API to a new
Include/cpython/longobject.h header file.

Move the following definitions to the internal C API:

* _PyLong_DigitValue
* _PyLong_FormatAdvancedWriter()
* _PyLong_FormatWriter()