ast+datum+macro: move r5js.Datum#isEqual to CompoundDatum.
datum: make r5js.Datum#cdrHelper_ private, to ease CompoundDatum transition.
ast+datum+misc: move r5js.Datum#getCdrHelper to CompoundDatum.
ast+datum: move r5js.Datum#setCdrHelper to CompoundDatum.
ast+datum: move r5js.Datum.cdrHelper_ to CompoundDatum.
ast+datum: move r5js.Datum#getQQLevel to CompoundDatum.
ast+datum: move r5js.Datum.qqLevel to CompoundDatum.