Packet

该功能的作用在于：把一个 byte数组打包，然后正确的解析回来
可用于网络，序列化等，支持嵌套（packet内部再放若干个packet）
用法如下

            Packet pack = new Packet(1,11,2,Encoding.Default.GetBytes("haha"));
            byte[] buf = pack.Pack();
          
            Packet pac = new Packet();
            pac.UnPack(buf, 0, buf.Length);
            PacketReader p = new PacketReader(128);
            p.Set(buf, 0, buf.Length);
            p.Set(buf, 0, buf.Length);

            List<Packet> ps = p.Get();
            Log.L(ps.Count);