Can't easily drive writev(2) from a DispatchData

In network programming it's often necessary to call writev(2) given a DispatchData . In fact, DispatchData seems a perfect data type to do that. Unfortunately, in Swift it's currently impossible to do so (in a guaranteed safe way).

Sure, there's DispatchIO.write but there's cases where we need to call writev directly so it should be supported.