Define Modality, prove that double negation is one
Add some theorems about equivalence of equiv and iso in set_cat
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.

Also move [equiv_path_sigma_hprop] to HProp.v, because it fits there,
and fits in nicely with the proof.
Robustify isequiv_path_sigma_hprop
Merge pull request #352 from JasonGross/ex-7-12

Define Modality, prove that double negation is one
Merge pull request #354 from JasonGross/sigma-hprop

Define a variant of [path_sigma] for hprops.
Merge pull request #357 from JasonGross/set-cat-morphisms

Add some theorems about equivalence of equiv and iso in set_cat