swift234

[stdlib]Replace NSMutableSet with Set

1. I think replace the NSMutableSet with Set<UIView>(and Set<NSView>)
could make the operations more Swift-like.

These code works fine in my Playground, and I also pass the test. But
I’m not quite sure it’s ok to use Set here rather than NSSet.

2. I also simplify the read-only computed property in “UIKit.swift”