Data.Graph patch

Here is a patch for Data.Graph which fixes an oversight in graphFromEdges. graphFromEdges returns a function which turns vertexes into nodes, but provides no way to go the other direction and figure out which vertex goes with a node. I simply return the already created function of type key -> Maybe Vertex in the tuple. I provided it as a new function graphFromEdges' and left the old functionality under the old name. if we don't care about backwards compatability, I'd recommend just replacing the one that is already there since it is a more complete interface and an easy transition for anyone currently using the library. John -- John Meacham - ⑆repetae.net⑆john⑈