Method traverse

crates/vm/src/object/core.rs:425–427

DO notice that call `trace` on `Py ` means apply `tracer_fn` on `Py `'s children, not like call `trace` on `PyRef ` which apply `tracer_fn` on `PyRef ` itself

(&self, tracer_fn: &mut TraverseFn<'_>)

    /// DO notice that call `trace` on `Py<T>` means apply `tracer_fn` on `Py<T>`'s children,
    /// not like call `trace` on `PyRef<T>` which apply `tracer_fn` on `PyRef<T>` itself
    fn traverse(&self, tracer_fn: &mut TraverseFn<'_>) {
        self.0.traverse(tracer_fn)
    }
}

unsafe impl Traverse for PyObject {