Add an implementation of Sliding Window

Sliding window holds `backlog` number of items, and sends itself to the
downstream every `period`.

It's backed by an array, items are written to the `pointer modulo array
size` position. When it's streamed down, tail is positioned before
tail, since it's historically correct.


Would that be something relevant/interesting for you guys? 
If yes, I could add some tests for it.