ochre-lang

Copperbox revision 3167.

I've removed Signal from the Value type in the intermediate syntax and added Tuple. Signal was a left over from when Ochre was embedded in Haskell. I've added Tuple even though tuples will need eliminating before the final translation to Csound. I believe I've devised an elimination scheme that will work if tuples are in the tail position - this will be easiest to do as a rewrite pass on the intermediate syntax (hence it needs tuples).