[Python-3000] optional argument annotations

[Tony Lownds]

> Obviously signature objects would grow support for annotations, but I
> still need the information to be carried on the code object to
> incorporate into signature objects.
>

Signature objects still need a way to know the nested parameters, right?
How about a co_argnames attribute? eg for

def f((x, y), z): pass

f.func_code.co_argnames would be (('x', 'y'), 'z')

I need to implement something like this to properly build  
func_annotations
inside MAKE_FUNCTION.

-Tony