v0.3 API removals

[Bromeon]

To remove:

1. Tween::new_gd()

   • expects create_tween()
   • see Discord thread
   • possibly extends to other classes such as SceneTreeTimer

2. Export for Gd

   • see Support OnEditor<T>/Required<T>for #[export] similar to OnReady<T> #892

3. ExtensionLibrary::override_hot_reload()

   • should follow a good default
   • could take into account the flag reloadable in .gdextension in the future (once that's accessible from Rust)

4. All deprecated APIs.

5. GdextBuild should maybe be changed into a module.

[Yarwin]

4. instance_from_id, is_instance_valid and is_instance_id_valid has been deprecated with Deprecate instance utilities in godot::global #901 and should be removed as well.

[Bromeon]

Good point, I think all deprecated APIs in the v0.2 cycle will be removed.

The last v0.2.x release will still contain the deprecations, and the migration guide (already WIP) recommends to update to latest patch before next minor, so that users get useful warnings instead of just missing symbols.