Add category methods on NSValue for geometry types

[1ec5]

MapKit has convenient category methods on NSValue for storing and retrieving CLLocationCoordinate2D (called “MKCoordinate” for some reason) and MKCoordinateSpan types. We should provide similar methods on NSValue for CLLocationCoordinate2D and MGLCoordinateBounds at the least. For CLLocationCoordinate2D, this is just a matter of moving some methods from osxapp into a file shared between the iOS and OS X SDKs.

/cc @friedbunny @tmcw