message: Use `map()`, rather than match on `Option<T>`
diff: @@ -565,16 +565,13 @@ where
  {
  type Item = (N, N, &'a E);
  fn next(&mut self) -> Option<Self::Item> {
  - match self.iter.next() {
  - None => None,
  - Some(b) => {
  + self.iter.next().map(|b| {
  let a = self.from;
  match self.edges.get(&GraphMap::<N, E, Ty>::edge_key(a, b)) {
  None => unreachable!(),
  - Some(edge) => Some((a, b, edge)),
  - }
  - }
  + Some(edge) => (a, b, edge),
  }
  + })
  }
  }
  @@ -606,10 +603,7 @@ where
  {
  type Item = (N, N, &'a E);
  fn next(&mut self) -> Option<Self::Item> {
  - match self.inner.next() {
  - None => None,
  - Some((&(a, b), v)) => Some((a, b, v)),
  - }
  + self.inner.next().map(|(&(a, b), v)| (a, b, v))
  }
  fn size_hint(&self) -> (usize, Option<usize>) {
  