Implement Sequence::sliding()

This PR implements a `sliding()` method which returns groups of a given size from the input:

```php
 // For arrays: [[1, 2, 3], [2, 3, 4], [3, 4, 5], [4, 5, 6], [5, 6, 7]]
Sequence::of([1, 2, 3, 4, 5, 6, 7])->sliding(3)
```

This can be used to calculate [moving averages](https://en.wikipedia.org/wiki/Moving_average)