Standalone kind signatures (#16794)

Implements GHC Proposal #54.

With this patch, a type constructor can now be given an explicit standalone kind signature:

{-# LANGUAGE StandaloneKindSignatures #-}
type Functor :: (Type -> Type) -> Constraint
class Functor f where
  fmap :: (a -> b) -> f a -> f b

This is a replacement for CUSKs (complete user-specified kind signatures), which are now scheduled for deprecation.

See the commit message for:

• User-facing changes
• Limitations
• Implementation notes