Merge uint and bigint

[andresilva]

Initially the code for big unsigned ints lived in this repo https://github.com/paritytech/bigint. At some point we copied the bigint code into https://github.com/paritytech/primitives. The problem is that we still kept committing to the original bigint repo. At this point the code in the bigint repo has seen some optimizations and is faster (u128 for limbs, dropped all assembly) and they're forgotten there. Now that bigint has moved once again, we should try to merge the current bigint code into this crate (and deprecate the bigint repo).