Add top-level Superview constraints prop

[tptee]

The new constraints prop on <Superview> allows adding constraints at the top level instead of at the child level. Useful for Victory integration and for consumer components that expose a constraints prop but not necessarily the names of their child subviews.

[coveralls]

Coverage decreased (-5.2%) to 84.483% when pulling 5d8a69a on feature-superview-constraints-prop into b8dfa38 on master.

[tptee]

Coverage decrease is because of isparta weirdness.