Provide derives for Neg and the *Assign traits.

[oli-cosmian]

These are equally straight forward as the other numeric impls, I'm guessing there just wasn't any need for these derives so far?

[cuviper]

Yeah, it's kind of weird because we don't really have derive(Add) etc., just derive(NumOps) which really implies the necessary ops. At least NumOps is specific to num-traits, but maybe we could also break out derives for all of the individual ops. It's a little bit of scope creep, but still in the general num domain.