Additional special functions

MPFR includes several useful special functions that would be nice to add, see: https://tspiteri.gitlab.io/gmp-mpfr-sys/mpfr/MPFR-Interface.html . For example, I recently needed the incomplete gamma function `mpfr_gamma_inc` which is available in MPFR >=4.0. Which versions of MPFR is this package trying to maintain compatibility with?

By the way, to assist users in adding functions with the ffi, it would be nice to either expose `Numeric.Rounded.Internal`, or expose the functions `unary` and `binary` in that module.