Array definitions: DI\link() in arrays

Splitting #190

We want to be able to use `DI\link()` in arrays:

``` php
return [
    'controllers' => [
        DI\link(AuthController::class),
        DI\link(OtherController::class),
    ],
];
```

Arrays could be traversed by PHP-DI so that _if_ they contain `DI\link()` entries, those are resolved.
