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`], but with a less powerful API.