Add sliding window sums

[treeowl]

slidingWindowSum
  :: (Monad m, Semigroup s)
  => Int
  -> Stream (Of s) m r
  -> Stream (Of s) m r

This could be used for things like precise sliding window means.

Rather than carrying around a Seq s, this would carry a finger tree-like structure with s annotations. I think a structure like Okasaki's implicit queues, augmented with annotations, will probably do the trick quite nicely; we shouldn't need anything quite as heavy as Hinze-Paterson 2–3 finger trees.

Sliding window min and max could actually use this too, but those can sometimes free memory more quickly with a specialized implementation.