ochre

Copperbox revision 2742.

I've added ref cells to Ochre. This will allow two useful functionalities:

1. Accessing global variables - needed for multi instrument control.
2. Preventing variables being optimized away (coalesced) - needed for delay lines.

Of course at the moment Ochre has nothing like register allocation so there is no current danger of variables being coalesced, but this is a compilation step it should do.