Mixer now receives a stereo bus
Rename new_user_synth_group to new_synth_group
Fix critical bugs in bus allocator:

* Hamster vectors need to be initialised before accessing with set
* Add missing param to reduce functions - surprising this worked at all!
Add initial (sketchy) pass of FX functionality (much more work needed):

General gist is:

    with_fx(:reverb, :room, 0.8) do
      play 50
    end

FX are also nestable for chained goodness...