code: func (b *ByteBuf) WriteUInt64(v uint64) (n int, err error) {
  	b.Expansion(8)
  	Uint64ToBytesTo(v, b.buf[b.writerIndex:b.writerIndex+8])
  	b.writerIndex += 8
  	return 8, nil
  }
docs: // WriteUInt64 write uint64 value to buf using big order
  // return write bytes count, error