sliding window
blocks of size elements, each is offset by step elements to the previous one

examples:
`(0..5).sliding 3 1`
=> `[[0, 1, 2], [1, 2, 3], [2, 3, 4], [3, 4, 5]]`

`(0..9).sliding 3 2`
=> `[[0, 1, 2], [2, 3, 4], [4, 5, 6], [6, 7, 8]]`