Expose new internal exponentiation primitives

This exposes the GMP functions `mpz_pow_ui()`, `mpz_powm()`, and
`mpz_invert()` as `powInteger`, `powModInteger`, and `recipModInteger`
respectively in the module `GHC.Integer.GMP.Internals`.

Signed-off-by: Herbert Valerio Riedel <hvr@gnu.org>
Expose GMP's `mpz_gcdext()` as internal primitive

The extended GCD computation is useful to have for implementing
algorithms such as the chinese reminder theorem.

Signed-off-by: Herbert Valerio Riedel <hvr@gnu.org>