p256::Scalar: implement Reduce<U512>

[survived]

I noticed that p256::Scalar does not implement Reduce<U512> trait. Other crates do have that: for instance, k256::Scalar does implement this trait. It makes it difficult to write generic code that uses this trait. Is there a reason for not implementing Reduce<U512> for p256::Scalar?

[tarcieri]

It's a missing feature: p256 lacks support for wide reductions.

crypto-bigint now contains a generic implementation of modular arithmetic which should make it somewhat straightforward to implement.