Prove Theorem 7.2.2 from the book

I chose to use the first proof because I couldn't find 4.7.7 already in
the library.  I didn't use [dpath_forall] (which HoTTBook.v tells me is
2.9.6) because that requires [Funext], and this lemma doesn't need it.
Merge pull request #347 from JasonGross/thm-7-2-2

Prove Theorem 7.2.2 from the book