Prevent duplication of unsafePerformIO on a multiprocessor

Prevent duplication of unsafePerformIO on a multiprocessor
Fixes #986.  The idea is to add a new operation

  noDuplicate :: IO ()

it is guaranteed that if two threads have executed noDuplicate, then
they are not duplicating any computation.

We now provide two new unsafe operations:

unsafeDupablePerformIO    :: IO a -> a
unsafeDupableInterleaveIO :: IO a -> IO a

which are equivalent to the old unsafePerformIO and unsafeInterleaveIO
respectively.  The new versions of these functions are defined as:

unsafePerformIO    m = unsafeDupablePerformIO (noDuplicate >> m)
unsafeInterleaveIO m = unsafeDupableInterleaveIO (noDuplicate >> m)