The Data instance for Ratio just uses the raw :% constructor and
doesn't check that the result is reduced to normal form.
The fix is to add back the Integral constraint on the Data
instance (which was dropped in c409b6f30373535) and to use % rather
than :% in the gfoldl and gunfold implementation.
This restores the invariant and matches the behavior of "virtual
constructors" we've used to patch up such problems elsewhere.
This addresses Trac #10011