Have custom type errors imply Void

This is a very minor issue, but TypeError (CustomTypeErrors) should entail a constraint like

import Data.Void

class (forall x. x) => No where
  no :: Void

so users don't have to use undefined or error ..:

instance TypeError (Text "Can't show functions") => Show (a -> b) where
  show :: (a -> b) -> String
  show = no & absurd