A deep nibble (4-bit integer) library.
[U4] is the actual nibble type, and offers common integer operations.
[U4x2] packs two [U4]s in a [u8], useful for building your own primitives.
[AsNibbles] allows you to view e.g [u8] as a slice of [U4]s.
[Stack] is Heap-optimised stack of [U4] which uses a packed
representation to half the memory usage of the equivalent [Vec<U4>], but
with a less powerful API.