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.
Rename Sliding Window to Moving Window
Add docstrings
Add for the Moving Window
Merge pull request #227 from ifesdjeen/feature/op/sliding-window

Add an implementation of Sliding Window