Add 'Relude.Extra.Newtype' module

[chshersh]

Just an experiment to have something like this:

class Newtype t where
    type Un t :: Type
    un   :: t -> Un t
    wrap :: Un t -> t
    -- un . wrap = id
    -- wrap . un = id

instance Newtype (Identity a) where
    type Un (Identity a) = a
    un = runIdentity
    wrap = Identity

newtype Bar = Bar Int
  deriving Newtype via (Identity Int)

And then we can write functions like these:

under :: Newtype t => (Un t -> Un t) -> t -> t
under f = wrap . f . un

[chshersh]

Okay, let's implement this via coerce. See my tweet:

• https://twitter.com/ChShersh/status/1026788941137997825