Proposal: Add concatMapM function (#2042)

[Don Stewart]

isaacdupree:
> Twan van Laarhoven wrote:
> >Hello library people,
> >
> >
> >I have noticed that many projects include a 'concatMapM' function:
> >
> >  concatMapM :: (Monad m) => (a -> m [b]) -> [a] -> m [b]
> >
> >I think this is useful in general, so let's add it to Control.Monad.
> 
> Me too.  (therefore I think it should be added.)
> 

(+1) seems reasonable.