Add a RemMixed trait and impl for Uints

[dvdplm]

Add a RemMixed trait in the same vein as SplitMixed and ConcatMixed, to calculate the remainder of differently sized integers.

Fixes #739.

[tarcieri]

This looks fine as a general direction

[dvdplm]

What else would you like to see done here, can you give me a rough idea?

[tarcieri]

@dvdplm if you think it's ready, can you remove draft?

[dvdplm]

In f6caf92 I added an impl for BoxedUint but not sure if that actually makes sense.

As a rule, should all traits be impl'd for Int as well as Uint, or are you handling that on a case-by-case basis?

[tarcieri]

FWIW we don't impl other *Mixed traits on Boxed* types.

Re: trait parity for Uint and Int, since the latter is something of a relatively new addition and not fully featured yet, we haven't been worrying about that, but it would probably be a good eventual goal.