Support ghc 9.6

[alexfmpe]

On top of #30

[alexfmpe]

I spent a couple hours adding traces and very confused on why a gazillion instances were being returned until I remembered about obsidiansystems/dependent-sum-template#7

Shouldn't the chunk of code that's equal between the two libs be re-used instead?

aeson-gadt-th/src/Data/Aeson/GADT/TH.hs

Lines 57 to 82 in 71a315a

	type family Skolem :: k -> k
	skolemize :: Set Name -> Type -> Type
	skolemize rigids t = case t of
	ForallT bndrs cxt t' -> ForallT bndrs cxt (skolemize (Set.difference rigids (Set.fromList (map tvName bndrs))) t')
	AppT t1 t2 -> AppT (skolemize rigids t1) (skolemize rigids t2)
	SigT t1 k -> SigT (skolemize rigids t1) k
	VarT v -> if Set.member v rigids
	then AppT (ConT ''Skolem) (VarT v)
	else t
	InfixT t1 n t2 -> InfixT (skolemize rigids t1) n (skolemize rigids t2)
	UInfixT t1 n t2 -> UInfixT (skolemize rigids t1) n (skolemize rigids t2)
	ParensT t1 -> ParensT (skolemize rigids t1)
	_ -> t
	reifyInstancesWithRigids :: Set Name -> Name -> [Type] -> Q [InstanceDec]
	reifyInstancesWithRigids rigids cls tys = reifyInstances cls (map (skolemize rigids) tys)
	-- | Determine the type variables which occur freely in a type.
	freeTypeVariables :: Type -> Set Name
	freeTypeVariables t = case t of
	ForallT bndrs _ t' -> Set.difference (freeTypeVariables t') (Set.fromList (map tvName bndrs))
	AppT t1 t2 -> Set.union (freeTypeVariables t1) (freeTypeVariables t2)
	SigT t1 _ -> freeTypeVariables t1
	VarT n -> Set.singleton n
	_ -> Set.empty

https://github.com/obsidiansystems/dependent-sum-template/blob/846df3885404b3c343453a2aa786eb6433a346d5/src/Data/GADT/TH/Internal.hs#L38-L63