Add `Enum (Down a)` instance that swaps `succ` and `pred`
IIUC from https://github.com/haskell/core-libraries-committee/issues/51#issuecomment-1104234948, I'm supposed to make a GHC MR for this proposed base library change.
IIUC from https://github.com/haskell/core-libraries-committee/issues/51#issuecomment-1104234948, I'm supposed to make a GHC MR for this proposed base library change.