Bala

Copperbox revision 163.

Added a new module OnsetQueue to Bala.Perform.

This is part of the work I've been doing on MIDI output. OnsetQueue represents events to be added to the MIDI stream at a future time. For example, if you have a high level representation of a note with a duration - if you render it to MIDI it will become a note-on at the current time onset, plus a note-off some time later. The onset queue stores these pending events to be queued later.

At present it doesn't recalibrate the onset time of the still outstanding events on a dequeue (the `upto` operation), this is most-likely wrong and no doubt will be apparent when I get more of the MIDI output code running.