impl Debug for more types

[geeklint]

Most types benefit from a Debug implementation, and it looks like only Rect supports it in this crate right now.

Specifically, the lack of the Debug trait for PackedItem means that you can't .unwrap() or .expect() the result of Packer::pack.

I think you should be able to add #[derive(Debug)] to all of {Item, PackedItem, PackedItems, Packer, and Rotation}. It looks like maybe you were avoiding it on the generic types but the derive macro should handle the generic type correctly for this crate's purposes.