Add Arrays::partition

Example:
```php
$input = ['a', 'b', 'c', 'd', 'e', 'f', 'g'];
$result = Arrays::partition($input, 4);
var_dump($result);
```
Would output:
```
array(4) {
  [0] =>
  array(2) {
    [0] =>
    string(1) "a"
    [1] =>
    string(1) "b"
  }
  [1] =>
  array(2) {
    [0] =>
    string(1) "c"
    [1] =>
    string(1) "d"
  }
  [2] =>
  array(2) {
    [0] =>
    string(1) "e"
    [1] =>
    string(1) "f"
  }
  [3] =>
  array(1) {
    [0] =>
    string(1) "g"
  }
}
```
One of these that has two non single groups would be good to guard against any remainderish bugs later.