Added miller-rabin primality test for bignums

[dsmurrow]

Pull Request Template

Description

I'm planning on adding an RSA implementation in the next few days. For that I needed an efficient primality test, and since we already had one implement I figured I'd use it, but I needed it to work on BigInts. At first I tried to use generics but I ended up needing like 7 trait bounds and it still didn't work so I bit the bullet and added another implementation that explicitly operates on BigInts.

Type of change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)

Checklist:

• I ran bellow commands using the latest version of rust nightly.
• I ran cargo clippy --all -- -D warnings just before my last commit and fixed any issue that was found.
• I ran cargo fmt just before my last commit.
• I ran cargo test just before my last commit and all tests passed.
• I checked COUNTRIBUTING.md and my code follows its guidelines.

Please make sure that if there is a test that takes too long to run ( > 300ms), you #[ignore] that or
try to optimize your code or make the test easier to run. We have this rule because we have hundreds of
tests to run; If each one of them took 300ms, we would have to wait for a long time.

[siriak]

Looks good, thanks!