Arg has unlawful instances

[David Feuer]

Foldable, Traversable, Bifoldable, and Bitraversable instances of
Data.Semigroup.Arg don't respect ==.

For example, Arg () 1 == Arg () 2 = True, but sum (Arg () 1) = 1 and sum
(Arg () 2) = 2.

I believe the best solution is to remove the Traversable and Bitraversable
instances, and define

    foldMap _ _ = mempty
    bifoldMap f _ (Arg a _) = a

There also needs to be some documentation about the fact that the Arg
constructor allows inspection that does not respect Eq.
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