Adaptor for consuming GDScript custom iterators

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It's possible to create and use custom iterators in GDScript, as documented on this page: https://docs.godotengine.org/en/3.1/getting_started/scripting/gdscript/gdscript_advanced.html#custom-iterators

An adaptor type that allows consuming these iterators with Rust's Iterator trait could be added. Additionally, an example for exposing a custom iterator could be created.