code: func (stream *Stream) WriteInt64(nval int64) {
  	var val uint64
  	if nval < 0 {
  		val = uint64(-nval)
  		stream.buf = append(stream.buf, '-')
  	} else {
  		val = uint64(nval)
  	}
  	stream.WriteUint64(val)
  }
docs: // WriteInt64 write int64 to stream