Add missing Monad/Traversable instances to tuples

[Francesco Ariis]

On Tue, Apr 02, 2019 at 08:53:10AM +0200, Henning Thielemann wrote:
> I think we discussed such instances at length in the past. Unfortunately, I
> cannot find the old discussion.

The discussion should be:

    https://mail.haskell.org/pipermail/libraries/2017-March/027824.html

I recall someone (most likely yourself) sensibly proposing instance
warnings before shipping this.

> Btw. what happened to GHC-Trac? Now I need JavaScript to see the comments.
> :-(

Alas, that ship has sailed too:

    https://gitlab.com/gitlab-org/gitlab-ce/issues/43433

-F