Make reify polykinded

Currently, we have

```haskell
reify :: forall a r. a -> (forall (s :: *). Reifies s a => Proxy s -> r) -> r
```

I believe we probably want something more like

```haskell
reify :: forall a r k. a -> (forall (s :: k). Reifies s a => Proxy s -> r) -> r
```

I have no particular opinion on the order of the type variables. Is there some fundamental reason this won't work? Is there some fundamental reason it's not desirable?