runtime: improve type annotations for r5js.CdrHelper.
runtime: move r5js.CdrHelper under runtime/ and lint.
runtime: remove r5js.CdrHelper#getCdrHelper.
macro: use r5js.Datum#setPayload instead of setting directly.
misc: strengthen signatures of two methods.
datum: remove r5js.Datum#isList.
datum: remove r5js.DatumType.DOTTED_LIST in favor of new r5js.Datum subclass.