Add bitwise xor and not

[kaidokert]

Mostly a copy from @Jperaire and/or implementation in #25

[tarcieri]

Needs rustfmt.

It seems I didn't notice the module names are inconsistent between limb and uint in #25. Anyway, I'd suggest making them consistent with the bit* prefix added in both cases. If you could do and and or as well for uint that'd be great.

Proptests would also be nice if you're interested in adding them. Otherwise I can merge without them and add them later.

[kaidokert]

I think i covered most of the feedback, but i can't find core::ops::Not implementation in num_bigint::BigUint, unsure as to how to write a clean proptest for it.

[tarcieri]

Thanks!