Tracking issue for `BoxedUint`/`BoxedResidue` missing features

[tarcieri]

I recently spiked out both of these types, but there are likely to be plenty of small gaps feature-wise.

This is a tracking issue for those gaps:

• Impl RandomMod for BoxedUint
• Trait for abstracting over *Residue types

cc @dignifiedquire @fjarri

[dignifiedquire]

• Comparisons between NonZero<BoxedUint> and BoxedUint
• Impl Hash for BoxedUint

[dignifiedquire]

• Impl Hash for NonZero<T>

[tarcieri]

Aside from the *Residue-like trait, which we can discuss in #431/#448, the Boxed* types are pretty feature-complete at this point. Closing.