datum: remove r5js.DatumType.LAMBDA by adding a r5js.Datum subclass.
datum: remove r5js.Datum#isProcedure in favor of direct instanceof checks.
datum: move r5js.Datum.name_ into Lambda subclass.
datum: remove r5js.data.newProcedureDatum in favor of r5js.Lambda ctor.
misc: replace newEmptyList() with r5js.SiblingBuffer ops where possible.