It's possible to use a more minimalist Magic

It strikes me as vaguely likely that something more like this formulation could have some efficiency advantages in some cases, as is usual for `Tagged` vs. `Proxy` based things of all sorts.

``` haskell
class Reifies s a | s -> a where
reflect2 :: Tagged s a

newtype Magic a r = Magic (forall (s :: *) . Reifies s a => Tagged s r)

-- I flipped this for minimalism; I don't really know how that affects efficiency.
reify2 :: forall a r . (forall (s :: *) . Reifies s a => Tagged s r) -> a -> r
reify2 k = unsafeCoerce (Magic k :: Magic a r)
{-# INLINE reify2 #-}

-- The usual interface can be implemented thus:

reflect :: forall s a proxy . Reifies s a => proxy s -> a
reflect _ = unTagged (reflect2 :: Tagged s a)
{-# INLINE reflect #-}

reify :: forall a r . a -> (forall (s :: *) . Reifies s a => Proxy s -> r) -> r
reify a k = reify2 tagged a
where
tagged :: forall (s :: *) . Reifies s a => Tagged s r
tagged = Tagged (k (Proxy :: Proxy s))
{-# INLINE reify #-}
```