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 could make the `_ = _ :> _` the default notation?  Or, perhaps better, we could put it in `long_type_scope` or something, and open that?