Simplify fixIO

Now that we have noDuplicate#, I think we can make fixIO much simpler, and perhaps also faster, using something like what we do for ST.

data Res a = Res (# State# RealWorld, a #)

fixIO f = IO $ \s ->
  let Res r@(# _s', a #) = unIO (noDuplicate >> f a) s
  in r

If f forks an IO thread demanding a, the noDuplicate should ensure that the computation is not re-entered, and the thread should wait for the main computation to complete.

So I believe, anyway. The comment about eager blackholing here is a bit vague and doesn't give an example.

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Trac field	Value
Version	8.0.1
Type	Task
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Core Libraries
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