ochre

Copperbox revision 2873.

I've added signal update and immutable top-level declarations that can't be let-eliminated or constant folded. The former allows the Csound optimization of minimizing variable declarations, the latter allows more readable generated code as i-rate declarations can be preserved.

Both implementations are hacks as I envisaged implementing them differently when I started designing Ochre. I'll probably have to rethink the Phantom layer somewhat as Signals now have a specific meaning to Ochre which the old code in the Phantom layer doesn't properly respect.