ast+datum+misc: move r5js.Datum#mapChildren to CompoundDatum.
ast+datum+misc: move r5js.Datum#replaceChildren to CompoundDatum.
datum+runtime: remove last use of r5js.Datum#appendChild and remove method.
ast+datum: move a clause of r5js.Datum#clone to CompoundDatum override.
ast+datum+misc: move r5js.Datum#firstSublist to CompoundDatum.
ast+datum: move a clause of r5js.Datum#resetDesugars to CompoundDatum.
datum+parse: simplify r5js.Datum#desugar by inlining a special case at call sites.