Add missing Monad/Traversable instances to tuples

[Sven Panne]

A strong -1 from me for the exact same reasons given 2 years ago:
https://mail.haskell.org/pipermail/libraries/2017-March/027883.html
Nothing has changed since then, we don't even have a warning flag yet (see
#11796). Just 2 remarks:

   * "Doesn't break existing code" is an invalid argument: Removing e.g.
type checking won't break existing code, either, but this is probably not a
worthy goal. And the proposal goes a step towards removing type checking in
a very subtle way.

   * I am not sure if consistent nonsense is better than partial nonsense.
:-/  (For a weaker formulation: sed 's/nonsense/surprising behavior/g')
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