Remove `Uniquable Int` instance, use `mkUniqueGrimily` directly instead

While going over code in UniqFM, I stumbled over code using getUnique :: Uniqable a => a -> Unique on Ints. And if you go to https://gitlab.haskell.org/ghc/ghc/-/blob/f78f001c91736e31cdfb23959647226f9bd9fe6b/compiler/GHC/Types/Unique.hs#L190, you really see

instance Uniquable Int where
 getUnique i = mkUniqueGrimily i

I really don't think this is a good idea. getUnique is used quite often, and almost never on Ints. So it's very surprising to see this instance! UniqFM uses it instead of mkUniqueGrimily, for example:

nonDetKeysUFM :: UniqFM key elt -> [Unique]
nonDetKeysUFM (UFM m) = map getUnique $ M.keys m

Very, very sneaky. It relies on the getUnique method to be an inverse to getKey :: Unique -> Int. But that doesn't necessarily have to be the case!

I think we should get rid of the Uniquable instance. Just use IntMap directly instead, if needed! If that's not an option, we should at least consider replacing those getUnique @Int calls by mkUniqueGrimily.