Return more specific type from RepeatedParser, etc.

[drhagen]

Right now, RepeatedParser, RepeatedOnceParser, RepeatedSeperatedParser, and RepeatedSeperatedOnceParser all promise to return a Sequence[Output]. This should promise the specific type returned, in case the caller wants it. This is is currently a list[Output], but we may want to change it to a tuple[Output, ...] when SequentialParser is changed to return a tuple.