Mementos are allowed to be null after deserialization--it's only when they can't be deserialized do we have a problem.
Ensuring null graveyard memento can't hurt the application.
Null mementos can be passed into a repository.
When deserializing a memento, even if it's a null value, we still know what it is.
Added ability to route a generic memento all the way into the routing table.
Routing table now provides the key to the restore static method.
Multiple mementos of the same type can now be registered--the first hydratable to "claim" a memento, owns it.