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 like the idea of putting it in a scope that can be opened (that would be useful in other contexts), but I don't think that would completely solve the problem here, because the user would still have to realize that `apply path_hprop` is inserting invisible coercions that could be made visible by opening that scope.