We can use the `ff::PrimeField::root_of_unity` method everywhere we
currently use this associated constant. If there is a more general
need for accessing this as an associated constant, we should consider
that for `ff::PrimeField`.
They already implement CofactorGroup (trivially, with the prime-order
subgroup being Self); this just enables Pallas and Vesta to be used in
cofactor-aware protocols that also want to leverage the affine point
representation.