Service Factories

Add the ability to define services from other factories. The biggest use case for this would be for doctrine entity repositories.

```php
$c->singleton(MyEntityRepository::class, function($c) {
    return $c->get('em')->getRepository(MyEntity::class);
});
```

vs.

```php
$c->serviceFactory(MyEntityRepository::class, 'em', 'getRepository', MyEntity::class);
```