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


If the `_` is necessary, that means I was wrong about `{IsHProp A}` being helpful, and you should put it back to the way it was before (`forall A, IsHProp A -> A + ~A`).  In some places, it means "make this argument implicit and maximally inserted, but apparently here it does nothing.