Fix MDL incompatibility with the order of associations. [ZILF-226]
This changes AssociationTable's enumerator to return associations in the same order as MDL, i.e. newest first.
The associations are stored in unordered dictionaries with unique GC behavior (ConditionalWeakTables), so rather than try to hack together an ordered equivalent, we just record a sequence number when an association is added, then sort them all by descending sequence number on retrieval.