`Group::Scalar` should be bound by `Field` and not `PrimeField`.

[carloskiki]

Issue moved from zkcrypto/ff#139 because it was in the wrong repository. You may read it first for context.

[str4d]

Group is intended to model a cryptographic group, not a mathematical group.

Cryptographic protocols by their nature require the group to have a large prime-order subgroup in which operations are performed. The PrimeGroup and CofactorGroup traits represent the fact that the group elements themselves can in some protocols end up being composite, but the scalar field in which the cryptographic protocols operate is always prime order. Hence, Group::Scalar: PrimeField.

[str4d]

@carloskiki if you feel strongly about this, the next step here would be for you to write a zkcrypto RFC for your proposed API change.