Move some of the methods and constants from `PrimeField` to `Field`

[carloskiki]

I feel like the split between Field and PrimeField is wrong. Field should encompass as much as possible what a field element is, while PrimeField should only contains things specific to prime fields.

Here is how I would like PrimeField to look like:

pub trait PrimeField: Field {
    const MULTIPLICATIVE_GENERATOR: Self;
    const S: u32;
    const ROOT_OF_UNITY: Self;
    const ROOT_OF_UNITY_INV: Self;
    const DELTA: Self;
}

Every method provided by PrimeField could sensibly be moved to Field, and constants like MODULUS, and CAPACITY could also be moved.

Is there a reason why this is not already the case?

[str4d]

@carloskiki if you want to move this forward, the next step here would be for you to write a zkcrypto RFC for your proposed API change.