Add slide to 07-map for declaring/creating maps (and change order?)

[stellentus]

I don't think there was a slide explaining how to initialize an empty map. We talked about it, but I didn't see it clearly on a slide. I think make would also belong on that slide.
But I wasn't paying close attention, so perhaps I missed it.

08-slices contained a bit more of that, so perhaps it was ok. I wonder if slices should come before maps. Perhaps maps could come after 10-methods, and the map exercise could involve working with a list of the BankAccount type. That would build on the students' pervious work and focus solely on map behavior in this exercise.

This should probably be two separate issues. But I'm clicking submit...