macro: r5js.ListLikeTransformer.Base_ should take a datum ctor, not a terminal.
read: r5js.read.bnf.Seq_ should take a datum ctor instead of a terminal "name".
misc: r5js.SiblingBuffer#toList should take a datum ctor, not a terminal.
macro: remove no-longer-used r5js.ListLikeTransformer.Base_#terminalForCtor_.
macro: remove a no-longer-needed dependency.
datum+macro+runtime: remove r5js.Datum#siblingsToList in favor of SiblingBuffer.
misc: rename r5js.List => r5js.ast.List.