Use trait to generalize over Uint and BoxedUint

[xuganyu96]

Per this draft PR, to migrate crypto-primes to accommodate both Uint and BxoedUint, a new trait UintLike will be defined in the crypto-bigint crate, and the functions in crypto-primes will take T: UintLike where it used to take Uint<const L: usize>. There are two major items:

• Define the UintLike trait to be used in crypto-primes
• Implement UintLike for crypto-bigint

In fact I think we can entirely do the trait definition and implementation in crypto-primes first, prove that it works, then move these traits to crypto-bigint.

Maybe the original maintainers are already working on this, but it still seems like a very fun project, so I want to give it a try, as well.