I'm happy with the API, except that --with-system-libmpdec is naturally
broken.  So I've to release a new libmpdec, which I'd rather do soon
because I don't want to revisit this later.

Antoine has looked at the API.  If anyone else has requests or objections,
now would be a good time to raise concerns.  These two functions are
the centerpiece of the PR:

   mpd_uint128_triple_t PyDec_AsUint128Triple(const PyObject *dec)
   PyObject *PyDec_FromUint128Triple(const mpd_uint128_triple_t *triple)