Add a kind synonym: MonadTransformer = (Type -> Type) -> (Type -> Type)

[sjshuck]

[original issue 83 by @Icelandjack]

-- The kind of monad transformers. When used, T :: MonadTransformer should serve as
-- an informal indicator that there is a MonadTrans T instance.
type MonadTransformer :: Type
type MonadTransformer = (Type -> Type) -> (Type -> Type)

Which appears all over the place, and simplifies a lot of messy kinds, MonadTrans reads fairly clear here and ComposeT (composition of monad transformers) does as well

type MonadTrans :: MonadTransformer -> Constraint

type ComposeT :: MonadTransformer -> MonadTransformer -> MonadTransformer
newtype ComposeT trans1 trans2 m a = ComposeT (trans1 (trans2 m) a)

From Elevator and others

type    Elevator :: MonadTransformer -> MonadTransformer
newtype Elevator trans m a = Elevator (trans m a)

type ReaderT :: Type -> MonadTransformer

type StateT :: Type -> MonadTransformer

type WriterT :: Type -> MonadTransformer

at very little cost (a single synonym).

[sjshuck]

MonadTransformer looks too similar to MonadTrans. Maybe a better name?

Also it's possible that something can have kind (Type -> Type) -> (Type -> Type) and not be a monad transformer. Natural transformations for example. Perhaps such a general kind synonym doesn't belong here.