Implement derives for generic wrapper types

[oli-cosmian]

fixes #19

[oli-cosmian]

Rebased

[oli-cosmian]

Just missed a couple spots.

oops, resolved and grepped to see if there are any others (there weren't).

[oli-cosmian]

Oh. I just realized that

#[derive(
    Debug,
    Clone,
    Copy,
    PartialEq,
    PartialOrd,
    ToPrimitive,
    FromPrimitive,
    NumOps,
    NumCast,
    One,
    Zero,
    Num,
)]
struct MyThing<T>(Wrapping<T>);

does not work, because there's no T: Num bound on the Num impl, causing rustc to fail to infer that Wrapping<T>: Num holds. If we add T: Num bounds, that makes struct MyThing<T, U>(SomeTypeImplingNumEvenIfTDoesNot<T>); not work anymore.

[cuviper]

because there's no T: Num bound on the Num impl, causing rustc to fail to infer that Wrapping<T>: Num holds. If we add T: Num bounds, that makes struct MyThing<T, U>(SomeTypeImplingNumEvenIfTDoesNot<T>); not work anymore.

Hmm, now it seems like we're in that gray area of https://github.com/rust-lang/rust/issues/26925, where it's not really obvious whether you should want bounds on the generic parameters or on the structural types. By adding #inner_ty: Trait, we're leaking that private type in a way that may not be desirable. I think we should probably only constrain the generic parameters, as the compiler does.

So far we've avoided this question because the newtype derivations are unconditional.