AsObjectArg<T> is now implemented for &mut Gd<T>

[Bromeon]

Based on #846.

If you have a &mut Gd, you can now pass it to engine APIs, just like &Gd.

[GodotRust]

API docs are being generated and will be shortly available at: https://godot-rust.github.io/docs/gdext/pr-847

[Bromeon]

Note that there's a problem with this:

fn exchange_semester(child: &mut Gd<Node>) {
    let mut parent: Gd<Node> = child.get_parent().unwrap();
    parent.remove_child(child);
    // child enjoys time abroad
    parent.add_child(child);
}

&mut T does not implement Copy, so the first child argument (of type &mut Gd) moves out, and the second occurrence is no longer valid.

error[E0382]: use of moved value: `child`
    |
    | fn exchange_semester(child: &mut Gd<Node>) {
    |                      ----- move occurs because `child` has type `&mut godot_core::obj::Gd<godot_core::classes::Node>`, which does not implement the `Copy` trait
    |     let mut parent = child.get_parent().unwrap();
    |     parent.remove_child(child);
    |                         ----- value moved here
    |     // child enjoys time abroad
    |     parent.add_child(child);
    |                      ^^^^^ value used here after move

The solution is simple, borrow as &T instead of &mut T, with &*:

fn exchange_semester(child: &mut Gd<Node>) {
    let mut parent: Gd<Node> = child.get_parent().unwrap();
    parent.remove_child(&*child);
    // child enjoys time abroad
    parent.add_child(child);
}

---

Other things like the recent AsArg addition are not implemented for &mut T exclusive references and T values, to encourage properly borrowing (and discourage premature clones). I wonder if this something to consider here too -- it would mean extra &* in some places.

So, in the cases where someone has a &mut Gd, they already need to use the &* pattern as soon as the value is passed to more than one function. Maybe it's better to make this explicit from the start than have different call syntaxes? A similar argument can be made for values and borrows 🤔

[Bromeon]

Reverted for now in #947; I'd like to have a more solid design before we support once-consumable &mut arguments in isolated places. It doesn't only affect Gd<T> but also GString, Variant etc.