Make reallyUnsafePtrEquality# levity-polymorphic

This patch gives reallyUnsafePtrEquality# the type

reallyUnsafePtrEquality# :: forall {l :: Levity} (a :: TYPE (BoxedRep l)). a -> a -> Int#

as per #17126 (closed).

I needed to update the containers submodule, as this change causes an issue with the definition

hetPtrEq :: a -> b -> Bool
hetPtrEq x y = isTrue# (unsafeCoerce reallyUnsafePtrEquality# x y)

(in fact triggering #18149 (closed)).