Int12 and Int4 Support

[sjkelly]

Some devices, such as the xrtrx, have native 12 bit support. There is the choice to extend to 16 bit or shift to 8 bit, either of which are typically used with bandwidth/precision tradeoffs.

It would be nice to have Int12 support so no such tradeoff has to be made in environments with a surplus of processing power, and where bandwidth can be saturated.

[sjkelly]

Fortunately it would be Complex{Int12}, so I think tagging the stream as ComplexInt12 will be fine for now. We aren't doing processing here.

[mbaz]

I'll leave this link here just in case: https://github.com/rfourquet/BitIntegers.jl