Proposal [Trac #1266]: Make Data.Graph.Inductive.NodeMap handle slightly messy input without crashing

See http://hackage.haskell.org/trac/ghc/ticket/1266 for the patch. The basic idea is that insMapNode should survive a duplicate node, and insMapEdge should survive (and insert) an edge whose endpoints weren't yet in the graph. This was impossible for the current type of insMapEdge, so I added a safeInsMapEdge instead. There's also a fairly small test suite in here, which people are welcome to suggest improvements to. I suggest a discussion period of two weeks, ending April 17, unless discussion is still active by then, in which case a week after it dies down. Thanks, Jeffrey Yasskin