Fix object leaks caused by unfulfilled yields

Now the stack saved in a `GDScriptFunctionState` is cleared as soon as the `yield()` operation is known not to be resumed because either the script, the instance or both are deleted.

This clears problems like leaked objects by eliminating cases of circular references between `GDScriptFunctionState`s preventing them and the objects they refer to in their saved stacks from being released. As an example, this makes using `SceneTreeTimer` safer.

Furthermore, with this change it's now possible to print early warnings about `yield()`s to released script/instances, as now we know they won't be successfully resumed as the condition for that happens. However, this PR doesn't add such messages, to keep the observed behavior the same for the time being.

Also, now a backup of the function name in `GDScriptFunctionState` is used, since the script may not be valid by the time the function name is needed for the resume-after-yield error messages.

modules/gdscript/gdscript_function.h

@@ -293,11 +293,12 @@ private:
 public:
 	struct CallState {
 
-		ObjectID script_id;
+		GDScript *script;
+		GDScriptInstance *instance;
 #ifdef DEBUG_ENABLED
+		StringName function_name;
 		String script_path;
 #endif
-		ObjectID instance_id;
 		Vector<uint8_t> stack;
 		int stack_size;
 		Variant self;
@@ -357,12 +358,18 @@ class GDScriptFunctionState : public Reference {
 	Variant _signal_callback(const Variant **p_args, int p_argcount, Callable::CallError &r_error);
 	Ref<GDScriptFunctionState> first_state;
 
+	SelfList<GDScriptFunctionState> scripts_list;
+	SelfList<GDScriptFunctionState> instances_list;
+
 protected:
 	static void _bind_methods();
 
 public:
 	bool is_valid(bool p_extended_check = false) const;
 	Variant resume(const Variant &p_arg = Variant());
+
+	void _clear_stack();
+
 	GDScriptFunctionState();
 	~GDScriptFunctionState();
 };