Add missing Monad/Traversable instances to tuples

[Henning Thielemann]

On Thu, 4 Apr 2019, Bryan Richter wrote:

> I also understand the consistency aesthetic, and I would support efforts 
> to make it feasible. The onus to "write a patch", though, is on the side 
> of people championing this aesthetic. :)

In Germany there is a law: If you damage environment in the course of a 
construction, you must compensate this somehow. E.g. if you fell a tree 
you must plant some new ones. Analogously, if you reduce type safety in a 
way then add compensation in another way. That is, I also think that it is 
the obligation of the instance proposers to also implement some instance 
warning. I would not be surprised if they find out this way that we have a 
case of YAGNI.