Definition:
>>> foldMapM f = Fold.lmapM f Fold.mconcat
Make a fold from a monadic function that folds the output of the
function using
mappend and
mempty.
>>> sum = Fold.foldMapM (return . Data.Monoid.Sum)
>>> Stream.fold sum $ Stream.enumerateFromTo 1 10
Sum {getSum = 55}