Refactoring and reorganisation of the scheduler
Change the way we look for work in the scheduler. Previously, checking to see whether there was anything to do was a non-side-effecting operation, but this has changed now that we do work-stealing. This lead to a refactoring of the inner loop of the scheduler. Also, lots of cleanup in the new work-stealing code, but no functional changes. One new statistic is added to the +RTS -s output: SPARKS: 1430 (2 converted, 1427 pruned) lets you know something about the use of `par` in the program.
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