proposal #2461: add Traversable generalizations of mapAccumL and mapAccumR

[Isaac Dupree]

Ross Paterson wrote:
> The proposal is to add the following functions to Data.Traversable,

>     mapAccumL :: Traversable t => (a -> b -> (a, c)) -> a -> t b -> (a, t c)

>     mapAccumR :: Traversable t => (a -> b -> (a, c)) -> a -> t b -> (a, t c)

It is useful as an education effort so that people can see how/which 
functions
can be naturally generalized -- or if converting code from using Lists.
(Probably it's useful in its own right, but I haven't used Traversable
recently enough to be able to report on that.) I often thought that even
Data.List.mapAccum[LR] is just on the edge of deserving to be its own 
function
seperate from foldl/foldr anyway...

-Isaac