Create separated shared library for the GMP wrapper

[skirpichev]

This adds also coverage tests for zz_* API

[skirpichev]

Oops. This breaks rebuilding of editable installation:

$ pytest tests/test_mpz.py --verbose -k to_str
ImportError while loading conftest '/home/sk/src/python-gmp/tests/conftest.py'.
tests/conftest.py:5: in <module>
    import gmp
.venv/3.14/lib/python3.14/site-packages/_python_gmp_editable_loader.py:311: in find_spec
    tree = self._rebuild()
           ^^^^^^^^^^^^^^^
.venv/3.14/lib/python3.14/site-packages/_python_gmp_editable_loader.py:347: in _rebuild
    raise ImportError(f're-building the {self._name} meson-python editable wheel package failed') from exc
E   ImportError: re-building the python-gmp meson-python editable wheel package failed