Fix required positional args

[Vipitis]

fixes #803

I think this is a fix, but it is pretty awkward. The parsing logic for Attribute simply couldn't really distinguish between struct field (dict members) and positional args for a function. because the required <-> optional logic is different between the two.

Maybe there should be another class for Parameter instead?

I didn't check if any of the implementations of the native api need fixes, because some seem to validate defaults twice already example while the vast majority simply never handled an incorrectly passed None.

Python still won't necessarily raise anything more useful and the error comes from the native layer, unless you enforce typing.