match_class! macro to dispatch subclasses

[Bromeon]

This declarative macro allows to dispatch a class to a variety of different subclasses, without tedious application of Gd::try_cast().

Basic usage:

let event: Gd<InputEvent> = some_input();

let simple_dispatch: i32 = match_class!(event, {
   InputEventMouseButton(btn) => 1,
   InputEventMouseMotion(motion) => 2,
   InputEventAction(action) => 3,
   _ => 0, // Fallback.
});

More diverse dispatch patterns are also supported:

let fancy_dispatch: i32 = match_class!(some_input(), {
    InputEventMouseButton(btn) => 1,

    // Block syntax for multiple statements:
    InputEventMouseMotion(motion) => {
        godot_print!("motion");
        2
    },

    // Qualified types supported:
    godot::classes::InputEventAction(action) => 3,

    // Fallback with variable -- retrieves original Gd<InputEvent>.
    // This pattern is equivalent to `InputEvent(original)`.
    _(original) => 0,

    // Alternatively, you can use `_` without a variable to discard the value.
});

[GodotRust]

API docs are being generated and will be shortly available at: https://godot-rust.github.io/docs/gdext/pr-1225