根据下标取值

关键代码如下：

func _getElement(    _ index: Int,    wasNativeTypeChecked: Bool,    matchingSubscriptCheck: _DependenceToken  ) -> Element {#if _runtime(_ObjC)    return _buffer.getElement(index, wasNativeTypeChecked: wasNativeTypeChecked)#else    return _buffer.getElement(index)#endif  }

_ContiguousArrayBuffer里的代码如下，只是取了一个指针的偏移。

internal func getElement(_ i: Int) -> Element {    _internalInvariant(i >= 0 && i < count, "Array index out of range")    return firstElementAddress[i]  }