Re-exporting a previous import should preserve previous OldOIDs

(Bug, Investigate , Priority: High, Test Status: No automated tests yet , Reported By Justin du Coeur, )
Import and Export
Summary: Currently, OldOID will only work through one export/import cycle. This can probably be improved, though.
It occurs to me that the big problem is that OldOID is an ExactlyOne. It could be a List, though, and the Import process could append to any previous values there.
This isn't quite trivial, since it makes the mapping process a bit more complex, but it should be terribly hard, either.
In order to make this worthwhile, the code in SpaceState that uses the OldOID would probably need a little enhancement, to pick up on all previous IDs.