[PROPOSAL] array_enkey()

Add `array_enkey()` function.
I like a lot Eloquent Collection `lists()` function and  `array_enkey()` has in a way the same approach.

For this array:
```php
// sample users
$users = array(
    array(
        'id'        => 'to',
        'firstname' => 'Taylor',
        'lastname'  => 'Otwell',
        'email'     => 'taylorotwell@gmail.com',
    ),
    array(
        'id'       => 'lm',
        'name'     => 'Lucas',
        'lastname' => 'Michot',
        'email'    => 'lucas@semalead.com',
    ),
);

// we choose to "enkey" this array using the email, and decide to preserve it
$preserved_keyed_users = array_enkey($users, 'email', true);

// printr_r($preserved_keyed_users);
array(
    'taylorotwell@gmail.com' => array(
        'id'        => 'to',
        'firstname' => 'Taylor',
        'lastname'  => 'Otwell',
        'email'     => 'taylorotwell@gmail.com',
    ),
    'lucas@semalead.com'     => array(
        'id'       => 'lm',
        'name'     => 'Lucas',
        'lastname' => 'Michot',
        'email'    => 'lucas@semalead.com',
    ),
);

// we "enkey" this array using the email, but we do not preserve it
$unpreserved_keyed_users = array_enkey($users, 'email', false);

// printr_r($unpreserved_keyed_users);
array(
    'taylorotwell@gmail.com' => array(
        'id'        => 'to',
        'firstname' => 'Taylor',
        'lastname'  => 'Otwell',
    ),
    'lucas@semalead.com'     => array(
        'id'       => 'lm',
        'name'     => 'Lucas',
        'lastname' => 'Michot',
    ),
);
```

**Note:**
This is a PR for 4.1 branch as this is not a breaking change.