added ``gf_pack_div``, used in ``dup_zz_hensel_step``
used ``gf_pack_div`` in ``_gf_pow_pnm1d2`` and ``gf_frobenius_map``
faster ``gf_frobenius_map`` using packing; ``gf_pack_rem``
ported ``ring_series.py`` and change in ``minimal_polynomial`` from PR 2630