Support levity polymorphic interface

Since GHC 9.4, various primitives (`Array#`, `SmallArray#`, `MutVar#`, ...) and their corresponding primops are now levity polymorphic. I propose making the `primitive` wrappers (`Array`, `SmallArray`, `MutVar`, ...) also levity polymorphic, so that users can take advantage of the levity polymorphism without having to use the primops from `GHC.Exts` directly.

This would also supersede [primitive-unlifted](https://hackage.haskell.org/package/primitive-unlifted) afaict.

I'm not sure if this would be a breaking change, but it should at least be minor, since type variables default to kind `Type` (so `Array a` would still be `Array (a :: Type)`).