Fix translations for unnamed function parameters

[joaotgouveia]

The current implementation of GetNamedDeclAsString defaults to returning self whenever the declaration corresponds to an unnamed function parameter. This leads to incorrect translations such as the following:

int main(int, char**) { return 0; }

fn main_0(self: i32, self: Ptr<Ptr<u8>>) -> i32 {
    let self: Value<i32> = Rc::new(RefCell::new(self));
    let self: Value<Ptr<Ptr<u8>>> = Rc::new(RefCell::new(self));
    return 0;
}

This patch addresses this issue by changing GetNamedDeclAsString to return self only when it is actually required, and _ otherwise.