Make Inherits<T> reflexive, with implied GodotClass bound

[Bromeon]

Previously, it was hard to express polymorphism such as "anything that is a Node", including both Node and derived classes (for example in function parameters).

This is now straightforward and also doesn't require a GodotClass bound anymore:

fn print_node<T>(node: Gd<T>)
    where T: Inherits<Node>
{
    let node = node.upcast(); // Gd<Node> inferred
    ... // use it
}

Also adds a couple of tests, as well as a minor panic improvement when invoking Godot from test/doctest.

[Bromeon]

bors r+

[bors]

Build succeeded:

• full-ci