Add missing Monad/Traversable instances to tuples

[amindfv at gmail.com]

> El 3 abr 2019, a las 02:59, Sven Panne <svenpanne at gmail.com> escribió:
> 
> 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.
> 

Another strong -1 from me for exactly this reason. I've seen too many bugs where a datatype was changed and suddenly e.g. "length" was being called with a ([x], y) instead of a [x], and we're without warning returning 1 every time.

I'd also very much appreciate work on the compiler flag for warning on an instance!

Tom
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