Support positional-only arguments in #[pyfunction]

[davidhewitt]

I noticed this morning that we don't yet have support for Python 3.8's positional-only arguments (PEP 570). This should be a nice standalone feature which anyone interested is welcome to implement. I can help mentor with any questions.

Some examples of how they interact:

>>> def foo(a, /, b):
...     print(a, b)
...
>>> foo(1, 3)
1 3

>>> foo(a=1, b=2)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: foo() got some positional-only arguments passed as keyword arguments: 'a'

# Also combines with keyword-only separator
>>> def foo(a, /, *, b):
...     print(a, b)
...
>>> foo(1, 3)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: foo() takes 1 positional argument but 2 were given

This will need changes in a couple of places:

• The proc macro code will need to be extended to support positional-only arguments. A starting-point would be to teach add_literal which already parses "*" to also parse "/".
• The runtime code to parse_fn_args will need adjusting to handle these positional-only arguments.

Both parts of the code are welcome to be refactored at the same time to deal with the additional complexity.

[davidhewitt]

The argument extraction was updated in #1440 so now we just need to add this to our macros!