Macro impl_rng 

macro_rules! impl_rng {
    ($type:ty $(, ($type_param:ty, $param_name:ty))?) => { ... };
}

Given a struct Alg which is a wrapper over [Field; N] for some N, implement Distribution<Alg> for StandardUniform.

As Distribution<Field> is implemented for StandardUniform we can already generate random [Field; N] elements so we just need to wrap the result in Alg’s name.