The castor::io::unmarshalUint64() method was not unmarshalling a 64-bit
integer in the same was as the legacy code of h/marshall.h.

The legacy code was treating a 64-bit integer as two 32-bit integers. It
would first unmarshall a 32-bit integer representing the lower powers of 2
and then marshall a 32-bit integer representing the higher powers of 2.

The castor::io::unmarshalUint64() method was incorrectly treating a 64-bit
integer as a whole by simply unmarshalling the whole 8 byte block of
memory in reverse byte order.