type family F a
   type instance F Int = Bool
   type instance F Bool = Char

In GHCi
   *TF> :kind (F Int, F Bool)
   (F Int, F Bool) :: *
   *TF> :kind! F Int
   (F Int, F Bool) :: *
   = (Bool, Char)

We could call it ":normalise" but it seemed quite nice to have an
eager version of :kind