Add SubResidue trait and impls for Residue and DynResidue

[fjarri]

Not sure about adding neg():

• Should there be a separate NegResidue trait, or should it be a part of SubResidue?
• The default implementation (via SubResidue) would require an access to zero, and therefore an additional trait bound, which DynResidue does not implement.

[tarcieri]

Should there be a separate NegResidue trait, or should it be a part of SubResidue?

A separate NegResidue trait seems fine to me

[fjarri]

Do you want me to add NegResidue in this PR, or leave it for another one?

[tarcieri]

Seems fine in this PR

[fjarri]

Speaking of, why are there separate *Residue traits at all? Wouldn't the core::ops ones suffice?

[tarcieri]

Yeah, I've had similar thoughts but never broached them. We should probably switch to the core::ops traits.

cc @jellevos

[tarcieri]

@fjarri perhaps you can follow up on switching to the core::ops traits and impl'ing Neg in that PR?

[jellevos]

Yeah, I've had similar thoughts but never broached them. We should probably switch to the core::ops traits.

cc @jellevos

Yes, in hindsight, that makes total sense. I saw the great progress you two made in the past weeks! Happy to help out if anything still needs to be taken care of regarding modular arithmetic.