Look into number-theory for faster primality and more number-theorectic functions (fastest in Rust as far as I know, the only short-coming is that it doesn't have sieving yet, although there are much faster ways to count primes).

List some of the functions/functionality you are looking for below. I've done a little bit of work in this, so there is a good chance I've already written it before or have some familiarity with it.