def each
      @references_to_keys_map.each do |rkey, ref|
        key = ref.object
        yield(key, @values[rkey]) if key
      end
    end
def each
      @references_to_keys_map.each do |rkey, ref|
        key = ref.object
        yield(key, @values[rkey]) if key
      end
    end
Iterate through all the key/value pairs in the map that have not been reclaimed
 by the garbage collector.