Improve phpArray asserter to test multi-dimensional array more easily.

Currently, if you want to test a multi-dimensional array, you should do something like that:
```php
$this
   ->array($array = $object->shouldReturnAnArray()))
   ->integer($array[0])->isEqualTo(42)
   ->string($array[1]['foo'])->isEqualTo('bar')
;
```
With this PR, you can do:
```php
$this
   ->array($object->shouldReturnAnArray()))
      ->integer[0]->isEqualTo(42)
      ->string[1]['foo']->isEqualTo('bar')
;
```
Moreover, i have adding an alias to the `phpArray` asserter in the asserter generator to improve readability.
So, you can do:

```php
$this
   ->in($object->shouldReturnAnArray()))
      ->integer[0]->isEqualTo(42)
      ->string[1]['foo']->isEqualTo('bar')
;
```