Support for multiple BACKs (ie. if you have multiple producers)

Also clean-up IrminQueue interface a little bit (with 'nice' ASCII arts)
Simplify the IO bits. Not sure the IrminIO.poll was really worth. I think I will simply use CStructs.
Use stock Cstruct.t instead of something which looks similar.

BufIO are now just mutable Cstruct.t (because that's easier to use when you do IO, instead of having to pass the global state around).