mapAccumL is a generally useful operation on Traversable containers. The only version currently offered uses (a copy of) lazy state to support streaming left-to-right consumption. This does not, however, seem terribly appropriate/efficient for other usage patterns (e.g., a FIFO queue with right view, or a Map). Could we add a version that uses a copy of Control.Monad.Trans.State.Strict to support those too?