Argument specification by function attribute can be ambiguous

[kngwyu]

For example, we can do

    #[args(a, "*", c = 3)]
    fn get&self, a: i32, b: i32, c: i32) -> PyResult<i32> { ... }

, where the position of separator is ambiguous.
And it's also problematic that here we can use unrelated idents like

    #[args(a, "*", d = 3)]
    fn get&self, a: i32, b: i32, c: i32) -> PyResult<i32> { ... }

[davidhewitt]

Fixed by the new #[pyo3(signature = (...)] option which replaces #[args] and doesn't allow for ambiguity.