datum: make r5js.Datum.desugars private
datum: make r5js.Datum.qqLevel private and provide a getter
datum: make r5js.Datum.cdrHelper private
datum: make r5js.Datum.immutable private
datum: make r5js.Datum.parent private and add getter+setter
all: make r5js.Datum.nextSibling private, adding getter+setter