Provide a mapRefAppend operation.

[dmkoppelman]

Kranc Issue 113 provided reference-like functionality to the MapLookup functions. In this issue add to this functionality an append function, mapRefAppend. Evaluating mapRefAppend[map,key,morevalue] will append morevalue to the current value of the reference corresponding to key in map. Note that key maps to a reference, and it's the target of the reference that's updated, so that the change is visible to all holders of the reference.