Warnings about missing signatures (local, kind, instance)

Motivation

Can we make a warning for missing signatures?

For

• local bindings
• standalone kind signatures
• instance signatures

Proposal

Create new warnings that will warn about missing local bindings, the name is up for grabs

{-# Options_GHC -Wmissing-local-signatures-all #-}

rev :: [a] -> [a]
rev as = rev'd where

  -- Local binding with no type signature:
  --   rev'd :: [a]
  rev'd = reverse as

Lack of kind-signatures

{-# Options_GHC -Wmissing-kind-signatures #-}

-- Type synonym with no kind signature:
--   type Option :: Type -> Type
type Option = Maybe

and instance sigs

{-# Options_GHC -Wmissing-local-signatures-all #-} 

type One :: Type
data One = MkOne

instance Eq X where
  -- Instance method with no kind signature:
  --   (==) :: X -> X -> Bool
  MkOne == MkOne = True