ast: put r5js.ast.List in its own file.
ast: put r5js.DottedList in its own file.
misc: rename r5js.DottedList => r5js.ast.DottedList.
proc: internal refactoring to make removal of r5js.Datum#forEach simpler.
datum+proc: migrate last remaining user of r5js.Datum#forEach and remove method.
ast: use override instead of type switch for r5js.ast.Quote#fixParserSensitiveIds.
misc: rename r5js.Quasiquote => r5js.ast.Quasiquote.
datum: make r5js.Datum#qqLevel protected, to simplify moving to subclass.
ast: put r5js.ast.Quasiquote in its own file.
ast: put r5js.ast.Unquote in its own file.
ast: put r5js.ast.UnquoteSplicing in its own file.