Impl std::num_traits::{One, Zero} for Scalar types

[Pythoner6]

I'm currently trying to use the ristretto Scalar type with nalgebra, but cannot do matrix multiplications, because nalgebra requires the type to implement One and Zero for that. Since we have Scalar::ONE and Scalar::ZERO already, it seems reasonable that maybe Scalar should implement the rust std traits for it (and I suppose maybe also std::num_traits::{ConstOne, ConstZero} too).