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 would make this a `Global Instance`, so that you don't have to give it explicitly in the proof of `if_not_hprop_then_equiv_Empty`.