export `pow_mod_fast` to the public

I think its good to export `pow_mod_faster` in the tests/rsa.c to the public API in the form `bignum_pow_mod` function.
Its complement `bignum_pow` but with fast modulo exponentiation functionality at the hand

Start with tests/rsa.c, where pow_mod_faster is mentioned, and compare it with the existing bignum_pow public API. Confirm the intended bignum_pow_mod entry point and verify that the fast modular exponentiation behavior is publicly available and covered by the relevant tests.

Written by the indexing model from the issue text.