ast: make r5js.ast.SimpleDatum.payload_ protected.
ast: add r5js.ast.CompoundDatum as a superclass for List/DottedList/Vector.
ast: make Unquote/UnquoteSplicing/Quasiquote subclass r5js.ast.CompoundDatum.
ast+misc: r5js.ast.Quote is a CompoundDatum, not a Literal.
ast+datum+misc: move r5js.Datum#forEachChild to CompoundDatum.