Extend LinkMap API to support retrieval of map keys

When using the LinkMap API there are times when it cannot be know in advance what keys will be used to store values in a map. It would be very useful to be able to retrieve a list of all keys for which values are currently entered. The following public methods of Helper would suffice

List<Object> links()

{return links("default"); }

List<Object> links(Object mapName)

{ /* return list of map keys */ }