Do exercise 6.9, add some minor lemmas about pointed types

The pointed types lemmas are in preparation for some other 6 exercises.
Merge pull request #336 from JasonGross/ex-6-9-pointed

Do exercise 6.9, add some minor lemmas about pointed types
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
Define a variant of [path_sigma] for hprops

In this case, we don't need to provide the second projection.

Also prove that it's an equivalence.
WIP
WIP
Add many [simpl rewrite] variants
WIP