Atom
在Swift中创建原子（线程安全）对象。objc.io Atomic的增强版本。
示例
let count = 10000
let atomic = Atomic([Int](0..<count))
DispatchQueue.concurrentPerform(iterations: count) { (i) in
let number = atomic.value[i]
atomic.mutate { $0.append(number + i) }
}