Put the smart_pointers exercise before the threads exercise

[luhem7]

While learning threading, you need to learn about the Arc type and why it is needed. Worse, there are no explicit exercises about the Arc type in the thread exercises. Immediately after the threading exercises, we do the smart_pointers exercises which do teach you about Cow, Rc and Arc types nicely with no prior dependencies on the threading exercise.

I suggest that we swap the order of the threads and the smart_pointers exercises so help the concepts flow better.