Added MotionKit ids to sugarcube_to_s

```
(nil)? tree
  0: . UIWindow(#a951c60, [[0.0, 0.0], [320.0, 568.0]])
  1: `-- UIView(#a76e950, [[0.0, 0.0], [320.0, 568.0]])
  2:     +-- UILabel(#a76f800, [[149.5, 273.5], [21.0, 21.0]], text: "Hi!")
  3:     `-- MyOtherLabel(#a8dc260, [[0.0, 0.0], [0.0, 0.0]], text: nil) motion_kit_id: :my_other_label
```

I could change it to just `id: :my_other_label` if `motion_kit_id:` is too long, but I was following the Teacup example above it.