Coding style: Using StandaloneKindSignatures in GHC

[Baldur Blöndal]

> encouraging the use of a standalone signature for type declarations where at least one parameter of the datatype does not have kind Type.

So Dict, Eq both get a sig but Fix and Either do not?

  type Dict :: Constraint -> Type
  type Eq   :: Type -> Constraint
  type Fix  :: (Type -> Type) -> Type

It's sensible to exclude tired tropes like `Type` and `Type -> Type`
but higher-order functors (like Fix) warrant a signature.

Caveat: The kind of type synonyms, type families and data families is
not necessarily determined by counting the syntactic arguments of X
like for a `data' declaration as Y could be a type, a functor, a
bifunctor..

  type X = Y