Port UnivalenceImpliesFunext

This closes #46.  Unfortunately, the typeclass glue won't work very
well until we migrate to the polyproj branch of HoTT/coq, because
typeclass resolution currently doesn't take into account universe
inconsistencies, so we'd need to explicitly provide two instances of
univalence to this theorem in order to use it.
Merge pull request #332 from JasonGross/ua-implies-funext

UA implies funext