(2nd attempt, this time leaving the `Constr` using `":%"`)

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 c409b6f3) 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 #10011

Reviewed By: ekmett, austin

Differential Revision: https://phabricator.haskell.org/D625