High-quality 64-bit checksum of the content of the object. Similar
to hashcode(), but a long to reduce the chance of hash clashes. For
composite objects this should be defined using the subcomponents' checksums
(or hashcodes if not available). If two Keyed objects have the same
checksum() there should be a 1 - 1/2^64 chance that they are the same
object by value.