- Finish bindings for other types:
  * ui (unsigned long int)
  * si (long int)
  * z  (mpz_t)
  * q  (mpq_t)
  * fr (mpfr)
  These types are similar to some Scheme types, but the FFI won't
  automatically translate q and z, for example. I'm not sure these
  are worth doing (but ui and si are!)

- Make these available:
  mpfr_set_default_prec
  mpfr_get_default_prec
  mpfi_set_prec
  mpfi_get_prec
  mpfi_round_prec