implement PartialOrd<Uint> for BoxedUint

[baloo]

TODO:

• Provide an implementation for BoxedInt

[baloo]

This came up in RustCrypto/signatures#906 but I'm not sure that's the right approach.

The alternative would be to duplicate the implementation of the subtle::ConstantTimeGreater and ConstantTimeLess, but I'm not sure I like this approach better than creating a short-lived BoxedUint.

Opinion?

[tarcieri]

@baloo I think the real goal of this is to make it possible to compare a Uint constant to a BoxedUint while avoiding boxing the former, which this doesn't accomplish.

It should be possible to have a generic core implementation of this functionality that operates over [Limb] (or a higher level type, i.e. #756) which can be reused for comparing Uint, BoxedUint, or a mixture thereof.

[tarcieri]

That said, if this came with TODOs and an open issue to optimize it, adding it now does open the door to future optimizations and enable code to be written with Uint constants, so I think it's OK for a start.

[tarcieri]

Err, sorry, my last comment meant this is fine as a start and we can optimize later.

It would be good to add TODOs for that though, and open a tracking issue

[baloo]

Oh that's wasn't about your comment, but I'd rather start over.
After looking at the code for a little while, I have actually no idea how to achieve an abstraction over [Limb]. That would not carry whether the outer type was actually a Uint or an Int, so I'm a bit lost, this is why I closed that PR.

[tarcieri]

@baloo we can leave that out for now. If the impls are in place they can be optimized later.

To answer your question, originally I used the name UintRef before changing it to LimbSlice, though that's a good point and it should still carry Uint in the name somehow. But in the interim it can just be function names.