Add convenience method to CGRect for getting position in a rect

[NickEntin]

We currently have a Position.point(in:) method to get the point at that position in the rect. This works well when you have the position already (e.g. as a parameter), but is a bit cumbersome when you don't, since it requires the full type.

let _ = Position.center.point(in: rect)

We should add a CGRect.point(at:) convenience method that calls through to this same logic, matching the existing UIView.point(at:) method.

rect.point(at: .center)

[NickEntin]

Now that we support layout directions, we'll also need a layoutDirection: UIUserInterfaceLayoutDirection parameter.