Replace unimplemented with todo

[randomPoison]

At some point we started using todo instead of unimplemented, but a couple places still use the old macro. May as well normalize on todo everywhere, since unimplemented is baaaaaaasically deprecated.

[fw-immunant]

It's fine to make things consistent everywhere, but since when is unimplemented! on the way out? I thought it was for functionality that isn't and shouldn't be implemented.

[randomPoison]

Oh fair point, I didn't think about unimplemented meaning "deliberately 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”.

So todo! still probably makes more sense for us to use in the exercise code, but unimplemented isn't deprecated in the way I thought.