message: [visit] add FilterNode impls for &FixedBitSet and &HashSet
  It's sometimes easier to pass in a reference to a bitset or hashset while
  using it as a temporary node filter. Previously the only other way to do
  this was to use a closure.
diff: @@ -47,6 +47,25 @@ where
  }
  }
  +// Can't express these as a generic impl over all references since that would conflict with the
  +// impl for Fn.
  +impl<N> FilterNode<N> for &FixedBitSet
  + where FixedBitSet: VisitMap<N>
  +{
  + fn include_node(&self, n: N) -> bool {
  + self.is_visited(&n)
  + }
  +}
  +
  +impl<N, S> FilterNode<N> for &HashSet<N, S>
  + where HashSet<N, S>: VisitMap<N>,
  +{
  + fn include_node(&self, n: N) -> bool {
  + self.is_visited(&n)
  + }
  +}
  +
  +
  /// A node-filtering graph adaptor.
  #[derive(Copy, Clone, Debug)]
  pub struct NodeFiltered<G, F>(pub G, pub F);
  