Parse and generate code for function parameters

It's only a list of <expression>s, how hard can it be? But we'll also need to implement pushing them onto the stack and popping the stack after the function returns.

How hard can it be? Quite hard, because a) the stack frame is 2 bytes bigger if there's a 'register' variable and b) parameters must be evaluated and pushed onto the stack in reverse order so that 'printf()' can be implemented. See 'test/functions/arity_var.c'.