Add missing Monad/Traversable instances to tuples

I submitted https://gitlab.haskell.org/ghc/ghc/merge_requests/644 but it might be still controversial. I'd like to get more opinions again. Here's the list of proposed instances: Foldable ((,,) a b) Foldable ((,,,) a b c) Traversable ((,,) a b) Traversable ((,,,) a b c) Functor ((,,) a b) (Monoid a, Monoid b) => Applicative ((,,) a b) (Monoid a, Monoid b) => Monad ((,,) a b) Functor ((,,,) a b c) (Monoid a, Monoid b, Monoid c) => Applicative ((,,,) a b c) (Monoid a, Monoid b, Monoid c) => Monad ((,,,) a b c) The absence of Traversable instances for tuples feels quite inconsistent given that there are Bitraversable instances already.