This PR introduces:

• GdResVec - a handy wrapper around Vec<Gd<T>> (where T: Inherits<Resource>), that can be used on Rust side in the same way as inner Vec<Gd<T>>, and is exposed on Godot side as Array<T>. It also inherits Property and Export
• serde_gd modules to handle collections of Gd<T>:
  • serde_gd::gd_resvec and serde_gd::ext_resvec for bundled and external serlialization of GdResVec<T>
  • serde_gd::gd_hashmap and serde_gd::ext_hashmap for bundled and external serialization of HashMap<K, Gd<T>>
• more robust testing with gd-rehearse