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


Nonblocking question: Instead of this, do we want to register as an `Instance` the fact that any function between contractible types is an equivalence, and then, to use it, put `contr_inhabited_hprop` in the context and pick up that `const tt` is an equivalence?