This PR was squashed before being merged into the master branch (closes #800).

Discussion
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Converters as service

This PR decouples the callback resolution service from `ServiceControllerResolver` in `CallbackResolver` so it can be used to resolve converter callback declared a service callbacks :

```php
$app->get('/user/{user}', function (User $user) {
    // ...
})->convert('user', 'service:method');
```

Commits
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0cdef8e5 Converters as service