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


How about just [renaming the coercion](https://github.com/mikeshulman/HoTT/compare/marcbezem:master...hprop-coercion-ex3.9)?  If you pull that, then `apply path_hprop` visibly changes `Unit_hp = hprop` into `hproptype Unit_hp = hproptype hprop`.