[5.0] Add ArrayAccess support to data_get

Mainly so that we can use it with `Eloquent` models and `Collection` objects:

```php
$comment = Comment::with('post.user')->get();

$user = data_get($comment, 'post.user');
```

This also goes nicely with the [recently added](https://github.com/laravel/framework/pull/6553) `where` and `contains` functionality to the collection class:

```
@if ($orders->contains('user_id', Auth::id())
    ......
@endif
```