code: func (b *ByteBuf) ReadByte() (byte, error) {
  	if b.Readable() == 0 {
  		return 0, nil
  	}
  
  	v := b.buf[b.readerIndex]
  	b.readerIndex++
  	return v, nil
  }
docs: // ReadByte read a byte from buf
  // return byte value, error