Enable overloading by signal-arity in the type system.

[hmc-cs-jbearer]

For example, we want a compress opcode which has 1 input and 1 output, and we want one which has 2 inputs and 1 output (for sidechain compression). CSound will let us do this, but the CSound++ type system as implemented will not let us map compress to "component of 1 or 2 inputs and 1 output`. We should enable this at least for built-ins (perhaps not for user-defined opcodes except where it is inherited from built-ins).

[hmc-cs-jbearer]

In the typechecker:

• As we go, associate with each bound identifier a list of types (rather than a single type)
• Every time we see a new definition, add it's type to the list.