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


I think that is something for the coq devs; there's been some work recently on the "cannot unify" error messages, so that, e.g., if the terms are identical, then Coq automatically tries to print more things.  What's the exact error message we get here?  (I'll submit or update or point to a relevant feature request, because I think the error message should say something the the effect of not being about to unify `Type` and `hProp` or `@paths Type` and `@paths hProp` or something.)