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


We'd need to rename the coercion anyway if we made `hProp` a sigma-type a la #514.