Define Append canonically

it's not a workhorse for composition, so no need to add
an extra argument

optics-core/src/Optics/Internal/Optic/TypeLevel.hs

@@ -73,8 +73,7 @@ type family Curry (xs :: IxList) (y :: Type) :: Type where
 
 -- | Append two type-level lists together.
 type family Append (xs :: [k]) (ys :: [k]) :: [k] where
-  Append '[]       ys  = ys -- needed for (<%>) and (%>)
-  Append xs        '[] = xs -- needed for (<%)
+  Append '[]       ys  = ys
   Append (x ': xs) ys  = x ': Append xs ys
 
 -- | Class that is inhabited by all type-level lists @xs@, providing the ability