message: Implement contains_edge() for StableGraph
diff: @@ -535,6 +535,14 @@ impl<N, E, Ty, Ix> StableGraph<N, E, Ty, Ix>
  }
  }
  + /// Lookup if there is an edge from `a` to `b`.
  + ///
  + /// Computes in **O(e')** time, where **e'** is the number of edges
  + /// connected to `a` (and `b`, if the graph edges are undirected).
  + pub fn contains_edge(&self, a: NodeIndex<Ix>, b: NodeIndex<Ix>) -> bool {
  + self.find_edge(a, b).is_some()
  + }
  +
  /// Lookup an edge from `a` to `b`.
  ///
  /// Computes in **O(e')** time, where **e'** is the number of edges
  