Big unsigned integer types

Macros to implement large-but-fixed sized unsigned integer types. The functions here are designed to be fast.

The crate builds and exports two commonly used types: U256 and U512. Other sizes can be constructed with construct_uint!(NAME, SIZE_IN_WORDS), e.g. construct_uint!(U128, 2);.

Run tests with cargo test --features=std,impl_quickcheck_arbitrary --release.