Another victim of the state hack

I have the impression that it's useful to you to see how often the state hack trips people up, so I've added some more comments to http://hackage.haskell.org/trac/ghc/ticket/2284 ...about how it bit me. This is a real headache. To be frank, controlling lazy evaluation and sharing to get the right complexity for your code can be hard enough at the best of times, without the compiler sometimes ignoring the sharing you try to ask for...