Implement Sequence::grouped()

This PR implements a `grouped()` method which returns groups of a given size from the input:

```php
Sequence::of([1, 2, 3, 4, 5, 6, 7])->grouped(3)  // [[1, 2, 3], [4, 5, 6], [7]]
Sequence::of(['one' => 1, 'two' => 2, 'three' => 3])->grouped(2)  // [['one' => 1, 'two' => 2], ['three' => 3]]
```