Use crate num-bigint in miller rabin

[hesampakdaman]

Changes

In this PR we will change from depending on u128 to BigUint. We will continue to do so until we only depend on this crate. Note that this change will make us tightly coupled to this crate (we provide no interface in between num_bigint and our program).

• Switch from u128 to BigUint from num_bigint crate in miller_rabin.
• Use references where applicable, esp. from the entrypoint to miller_rabin and downstream. Later we will change the trait PrimalityTest to take &BigUint instead of u128.