Prove [IsHSet hProp]

This is required to prove that the precategory of [hProp]s is strict.

We put the lemmas in `types/Universe.v` because we can't [Require Import
types.Universe.] in `HProp.v`, because `types/Universe.v` depeneds on
`HProp.v`.
Fix the module names for functor and NT

Before, e.g., [Functor.sum] wouldn't have worked.  Now it does, because
we use [Include] rather than [Export].
Implement Functor.Prod
Export functor product notations
Port natural transformation products
Add an explicit lemma for [path_path_sigma]
Remove the [repeat match goal]

We only use each lemma one time, so we don't need it here.
Merge pull request #226 from JasonGross/trunc-hprop

Prove [IsHSet hProp]
Merge pull request #229 from JasonGross/functor-nt-product

Port products of functors and natural transformations