Add missing Monad/Traversable instances to tuples

[Henning Thielemann]

On Tue, 2 Apr 2019, Fumiaki Kinoshita wrote:

> 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.

I think we discussed such instances at length in the past. Unfortunately, 
I cannot find the old discussion. At least here is a reddit discussion:
    https://www.reddit.com/r/haskell/comments/3oq0kd/proposal_eliminate_the_monad_implementation_on/

If I would use such an instance in my code this would be an accident and a 
programming error. That's why I would want to have instance warnings 
first:
    https://gitlab.haskell.org/ghc/ghc/issues/11796


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