Adjunction f u means f a is isomorphic to (a, Rep u)

There is an explicit isomorphism for this:

``` haskell
fromAdj :: Adjunction f u => f a -> (a, Rep u)
fromAdj = rightAdjunct (tabulate . (,))

toAdj :: Adjunction f u => a -> Rep u -> f a
toAdj = index . unit
```

And there should be a newtype wrapper, for instance named `Adj`, that works like `Co` in Data.Functor.Rep, and provides instances for Comonad, instances for Applicative and Monad when `Rep u` is a Monoid, etc. Also, there would be an Adjunction between `Adj f` and `Co u`, so that `Co u` has an Adjunction whenever u does.