Note [Representing TyCon kinds: KindRep]

One of the operations supported by Typeable is typeRepKind,

    typeRepKind :: TypeRep (a :: k) -> TypeRep k

Implementing this is a bit tricky for poly-kinded types like

    data Proxy (a :: k) :: Type
    Proxy :: forall k. k -> Type

The TypeRep encoding of `Proxy Type Int` looks like this:

    $tcProxy :: GHC.Types.TyCon
    $trInt   :: TypeRep Int
    TrType   :: TypeRep Type

    $trProxyType :: TypeRep (Proxy Type :: Type -> Type)
    $trProxyType = TrTyCon $tcProxy
                           [TrType]  -- kind variable instantiation
                           (tyConKind $tcProxy [TrType]) -- The TypeRep of
                                                         Type -> Type

    $trProxy :: TypeRep (Proxy Type Int)
    $trProxy = TrApp $trProxyType $trInt TrType

    $tkProxy :: GHC.Types.KindRep
    $tkProxy = KindRepFun (KindRepVar 0)
                          (KindRepTyConApp (KindRepTYPE LiftedRep) [])

Note how $trProxyType cannot use 'TrApp', because TypeRep cannot represent
polymorphic types.  So instead

 * $trProxyType uses 'TrTyCon' to apply Proxy to (the representations)
   of all its kind arguments. We can't represent a tycon that is
   applied to only some of its kind arguments.

 * In $tcProxy, the GHC.Types.TyCon structure for Proxy, we store a
   GHC.Types.KindRep, which represents the polymorphic kind of Proxy
       Proxy :: forall k. k->Type

 * A KindRep is just a recipe that we can instantiate with the
   argument kinds, using Data.Typeable.Internal.tyConKind and
   store in the relevant 'TypeRep' constructor.

   Data.Typeable.Internal.typeRepKind looks up the stored kinds.

 * In a KindRep, the kind variables are represented by 0-indexed
   de Bruijn numbers:

    type KindBndr = Int   -- de Bruijn index

    data KindRep = KindRepTyConApp TyCon [KindRep]
                 | KindRepVar !KindBndr
                 | KindRepApp KindRep KindRep
                 | KindRepFun KindRep KindRep
                 ...