This fixes most of the performance regression in these functions
between 6.8.3 and 6.10.1, it was due to a slight difference in the
strictness between the old and new versions (see comments for
details).  There's still a few percent loss in performance in the div
test in nofib/spectral/integer that I haven't tracked down as yet.

This means that we can export the constructors, but still keep the
GHC.Integer interface generic.

if on a 32-bit machine