The `Refl` constructor had previously been re-exported from
`Data.GADT.Compare`.
This also replaces the dependency on `dependent-sum` with `some`,
where the `Data.GADT.Compare` module is now defined.
Tested by building locally with GHC 8.4.4, 8.6.5 and 8.8.3 in `cabal`.
Fixes#585.