Another victim of the state hack

[Jules Bean]

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...