misc: remove r5js.DatumType.MACRO in favor of a new r5js.Datum subclass.
misc: remove r5js.newMacroDatum in favor of r5js.MacroDatum ctor.
misc: remove r5js.DatumType.REF in favor of a new r5js.Datum subclass.
misc: remove r5js.Datum#maybeDeref in favor of instanceof + r5js.Ref#deref.