Default values for function parameters should allow non-literals

[yeroca]

I would like to have the ability to use named constants for function parameter default values, e.g.:

defaultXSlots = 12
defaultYSlots = 4

fn create1dGrid(numXSlots? :number(_) = defaultXSlots) { ... }

fn create2dGrid(numXSlots? : number(_) = defaultXSlots, numYSlots? :number(_) = defaultYSlots) { ... }

Currently the syntax forbids non-literal defaults.

[jtran]

I would like to be cautious about adding arbitrary expressions, but I think extending to variables would be a good start.

In terms of implementation, we need to evaluate at call time, not function definition time. That's a big mistake that Python made.

If KCL were totally immutable, it wouldn't matter. But we do have some forms of mutation. So we should be careful to do it right.