message: BUG: Fix off by one in StableGraph::node_bound
  It was 1 larger than it should, which is no big deal in some context.
  But it wasn't as correct as was intended. A graph without holes has
  .node_count() == .node_bound();
diff: @@ -1160,7 +1160,9 @@ impl<N, E, Ty, Ix> NodeIndexable for StableGraph<N, E, Ty, Ix>
  {
  /// Return an upper bound of the node indices in the graph
  fn node_bound(&self) -> usize {
  - self.g.nodes.iter().rposition(|elt| elt.weight.is_some()).unwrap_or(0) + 1
  + self.node_indices()
  + .next_back()
  + .map_or(0, |i| i.index() + 1)
  }
  fn to_index(&self, ix: NodeIndex<Ix>) -> usize { ix.index() }
  fn from_index(&self, ix: usize) -> Self::NodeId { NodeIndex::new(ix) }
  