feat(core): support easier, type-safe deferred calls

Author: goatfryed

itest/rust/src/object_tests/deferred_call_test.rs

@@ -11,7 +11,7 @@ use crate::object_tests::deferred_call_test::TestState::{
 use godot::obj::deferred::{DeferredEngineCallable, DeferredUserCallable};
 use godot::prelude::*;
 use godot::task::{SignalFuture, TaskHandle};
-use std::ops::{Deref, DerefMut};
+use std::ops::DerefMut;
 
 #[derive(GodotConvert, Var, Export, Clone, PartialEq, Debug)]
 #[godot(via = GString)]