BigInteger module missing routines?

[lydia-duncan]

We provide bigint.fac() but we don't provide the double factorial or multi-factorial versions. GMP has them in its documentation and they are in the GMP module but we don't have a bigint version. Should we?

mpz_primorial_ui() is also not covered. Should it be? (It is in the GMP module)

Note: the definitions of them in GMP can be found here: https://gmplib.org/manual/Number-Theoretic-Functions

I lean towards yes (and Michael does as well)