use a more rust idiomatic way of handling nth_value #448

I wonder if a more idomatic Rust way of doing this would be an enum like

enum NthValue {
  First,
  Last,
  Nth(u32)
}

And then in NthValueAccumulator::scan you would have something like

match self.n {
  NthValue::First|NthValue::Nth(1)  => {...}
  Nth::Last => {..}
}

There is nothing wrong with the special value approach either -- I just figured I would point it out

Originally posted by @alamb in #403 (comment)