Use NSHashTable with for .. in .. style enumeration with Swift2 (safely)

NSHashTable+SequenceType.swift

extension NSHashTable: SequenceType {
  
  public func generate() -> AnyGenerator<AnyObject> {
    
    var array = self.allObjects
    var nextIndex = array.count-1
    
    return anyGenerator {
      if (nextIndex < 0) {
        return nil
      }
      return array[nextIndex--]
    }
  }
}