WriteMessages supports sending messages using iter

[lazychanger]

old:

func main {
  total := 1e6

  msgBuf := make([]Message, 200)
  for i:1;i<=total; i++ {
    if i % 200 ==0 {
       conn.WriteMessages(msgBuf...)
       clear(msgBuf)
    }
    msgBuf[i%200] = SomeMessage
  }
}

iter:

func main {
  total := 1e6
  conn.WriteMessagesByIter(func(yield func(Message) bool) {
     for i:1;i<=total; i++ {
       if (!yield(SomeMessage)) {
          return
       }
     }
  })
}