Lemmas 3.3.2 and 3.3.3 and Exc. 3.9

Lemmas 3.3.2 and 3.3.3 added to HProp
Backrefs added to HoTTBook
Solution to Exc.3.9 added to HoTTBookExercises


I'm very confused.  There's no error message in the existing proof; it's just that it's confusing that `apply path_hprop` doesn't seem to change the goal.