[Bug] Positional and optional arguments not properly distinguished by runtime

[softmattertheory]

Describe the bug
A function call with optional arguments may occasionally match an implementation with positional arguments.

To Reproduce
A minimal example that runs and shouldn't:

fn f(x,y,z,a) { return -1 }

print f(1,z=3)

Expected behavior
The above example should generate an error e.g. 'InvldArgs'

Additional context
The underlying problem is that the CALL opcode doesn't specify the number of positional arguments, so the runtime infers them if it is expecting optional arguments.