Move various operations on Words to a new primitves module

[fjarri]

TODO: review function names

[tarcieri]

I guess this is draft because it's WIP? Otherwise looks great so far

[fjarri]

Adjusted the names and added some comments.

[tarcieri]

This seems good unless you wanted to add the Hacker's Delight overflow checks and such

[fjarri]

If they're used elsewhere other than const_choise.rs we can add them, but for now I think this is good enough.

[tarcieri]

There is duplication between const_choice.rs and modular/boxed_residue/mul.rs

[fjarri]

Not anymore.

[tarcieri]

It seems like these two duplicate each other:

https://github.com/RustCrypto/crypto-bigint/blob/00e1a60/src/const_choice.rs#L71-L85

And it's also duplicated here:

https://github.com/RustCrypto/crypto-bigint/blob/00e1a60/src/modular/boxed_residue/mul.rs#L305

Potentially all three could call a common function?

[fjarri]

The one in boxed_residue/mul has been removed in this PR, replaced with sbb(). The one in const_choice can indeed be expressed through the existing method, fixed. The u32/Word repetitions can be removed with a macro, but I'm not sure if it's worth it.

[tarcieri]

Nice! Looks great now