refactor(box1): prefer todo! over unimplemented!

[Lioness100]

todo! better matches the exercise's intent than unimplemented!

The difference between unimplemented! and todo! is that while todo! conveys an intent of implementing the functionality later and the message is "not yet implemented", unimplemented! makes no such claims. Its message is "not implemented". Also some IDEs will mark todo!s.

Furthermore, all other exercise's use todo!, so it's also a matter of consistency.