Add missing Monad/Traversable instances to tuples

[Ruben Astudillo]

On 02-04-19 03:16, Ganesh Sittampalam wrote:
> Presumably these would leave us with things like length (2,3,4) = 1 ?
> 
> If so I'm a weak -1; it's awful but the ship has already sailed with
> pairs, so maybe consistency beats more "wat" moments.

Exactly, this is specially confusing when the tuple is of different
types `(a,b)`, as common examples with ghci on `(a,a)` will return the
"correct" value for `length`. -1 for me too.

-- 
-- Ruben
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