Do most of Exercise 4.5

I'm still not sure how to give a higher-level proof of [IsEquiv f ->
IsEquiv (ap f)].  Also, I'm not sure if the [Proof with try
typeclasses eauto.] and [...] are appropriate for HoTTBookExercises.
Comment on use of [Proof with] and [...]
Merge pull request #319 from JasonGross/ex-4-5

Do most of Exercise 4.5