Generic Diff Function

[axrs]

(defn diff-by
  "Returns a map of :added, :changed, :removed, and :same"
  ([aggregator existing updated] (diff-by aggregator = (fn [_ right] right) existing updated))
  ([aggregator comparator existing updated] (diff-by aggregator comparator (fn [_ right] right) existing updated))
  ([aggregator comparator changed-merger existing updated]))