ochre

Copperbox revision 2780.

I've added List to the type representation and made tuples heterogeneous so I can model Csound's C-style arg lists - the calling convention for Csound opcodes is like C but Ochre can now encodes variadic functions as lists. As nothing really uses source language types within Ochre (save temporary variable generation), having polymorphic lists and tuples doesn't add complexity.

Also I've made a start on signal modifiers.