Implement Python unions

[crisidev]

Motivation and Context

Continue work on #1367.

Description

Implement support for the union shape in the Python server.

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[unexge]

Wonder if we can leverage type narrowing here.

So we generate the following:

#[pyo3]
struct Square;

#[pyo3]
struct Circle;

// :type typing.Union[Square, Circle]:
enum Shape {
    Square(Square),
    Circle(Circle),
}

impl IntoPy<PyObject> for Shape {
    fn into_py(self, py: Python<'_>) -> PyObject {
        match self {
            Shape::Square(s) => s,
            Shape::Circle(c) => c,
        }
    }
}

impl FromPyObject<'_> for Shape {
    fn extract(obj: &PyAny) -> PyResult<Self>{
        if let Ok(s) = obj.extract::<Square> {
            Ok(Shape::Square(s))
        } else if let Ok(c) = obj.extract::<Circle> {
            Ok(Shape::Circle(c))
        } else {
            Err("invalid shape")
        }
    }
}

and users can write:

# We will generate this type-stub for enums:
Shape = Square | Circle

def stringfy(shape: Shape) -> str:
    if isinstance(shape, Square):
        return "■"
    # Here type-checker knows `shape` must be `Circle`
    return "●"

I believe with Python 3.10 they can even write:

def stringfy(shape: Shape) -> str:
    match shape:
        case Square:
            return "■"
        case Circle:
            return "●"
        case _:
            raise NotReachable()

🤯

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[crisidev]

Wonder if we can leverage type narrowing here.

So we generate the following:

Unfortunately that is not enough as it will not cover nested unions like

union Something {
    member_a: AnotherUnion
    member_b: Integer
}

union AnotherUnion {
    member_a: Integer
    member_b: String
}

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[unexge]

Thank you for doing this 🎉

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