Return packed byte representation of the ARP header.

def to_bytes(self):
        '''
        Return packed byte representation of the ARP header.
        '''
        return struct.pack(Arp._PACKFMT, self._hwtype.value, self._prototype.value, self._hwaddrlen, self._protoaddrlen, self._operation.value, self._senderhwaddr.packed, self._senderprotoaddr.packed, self._targethwaddr.packed, self._targetprotoaddr.packed)