polyval, ghash: Add flavors for Little Endian and Big Endian

[makavity]

According to this issue, bundled ghash should be removed from belt-dwp crate.

Could you clarify your preferred approach for this?
I suggest, we should add generic param for arithmetic, like in CTR crate.

@newpavlov

[newpavlov]

I think we can start we the naive approach of introducing a generic parameter for byte order to see whether it would be sufficient for belt-dwp and mgm. But I also wonder if we could also use it to abstract over differences between polyval and ghash.

[tarcieri]

FWIW I plan on refactoring those crates soon, and introducing FieldElement types which will hopefully make these more exotic use cases easier