Make BBBuffer generic over the index variables

[jamesmunns]

It would be nice to use atomic types smaller than usize for tracking variables. This could be a significant size savings for buffers like BBBuffer<U128> (18 bytes vs 6 bytes overhead on a 32-bit target).

This should be possible by following the tricks used by heapless, and could be made backwards compatible to alias current constructors to use usize as the generic parameter.

[jamesmunns]

This would also help in making this mergable into heapless, CC rust-embedded/heapless#65

[jamesmunns]

CC @korken89, who I think is better at generic shenanigans that I am :)

[korken89]

Yeah, this is no problem to do, ping me if you want some help :)