Selective packing of relations

Add an option to only pack those relation table entries for which the 'from' entry is packed as well. As a(n intended) side-effect of the implementation, the order of the entries in packed relations should now also be stable, provided the 'key' of the 'from' entries is stable.

Fixes the underlying problem which results in the symptoms described in DaVinci#96 (closed). Closes DaVinci#96 (closed)