Some minor edits to the loops lesson

I added a challenge problem that introduces while loops--these don't seem to be critical to the lesson material but they are an important element of programming and it seems useful to at least mention them here. I also reworded some of the explanation/discussion of mutable vs. immutable variables, in an effort to improve clarity. Finally, I made a couple of other (very minor) changes to some examples.
could add brief caution that "while" loops can become infinite (preventing program completion) if condition is never met.