Introduce new trait for integers in field module

[divergentdave]

This introduces a new Integer trait that accompanies FieldElementWithInteger. Most of the trait bounds on the integer type are moved to be super-traits of Integer, related error associated types are moved into the trait, and the zero_integer() and one_integer() static methods are renamed and moved in as well. This is a follow-up to #819, to make breaking changes I deferred then.

[inahga]

FWIW some boilerplate can be elided with a default implementation like so

    fn zero() -> Self {
        0.try_into().unwrap()
    }

but that doesn't really seem like much of an improvement. I might feel better about it if we could also kill the type TryInto... boilerplate with default associated types, but that's an unstable feature.

[divergentdave]

The associated types for errors aren't strictly necessary, it would be possible for users to write something like <F::Integer as TryFrom<usize>>::Error instead of F::TryFromUsizeError, but the associated types as pseudo-aliases makes things easier to read, so I'm okay with keeping them around. I think I'd prefer to keep zero() and one() as required methods, and just use literals in each impl block, since it feels less hacky.