Aul: Allow '...' as final parameter in func decls

Accepting an ellipsis as the final parameter in a function declaration
makes it immediately obvious that the function can take a variable number
of parameters
Aul: ExtraWarn when turning a fn varargs in body

Par() calls and '...' in function calls make functions take varargs
without this being obvious from the header. ExtraWarn about this so people
who care about it can add an ellipsis to the parameter list.