Narrow integers #6

Would be cool to have a NarrowIntegers.jl package, generalizing integers to sub-byte widths like how Microfloats.jl generalizes floats.
These narrow integers could be implemented with UInt8 with a mask that gets applied after each operation Then there could be a BitPacking.jl extension that implements efficient broadcasting methods that handles overflows. See https://julialang.org/blog/2018/05/extensible-broadcast-fusion/.
Could maybe use BitIntegers.jl to do bit-manipulaton on chunks.