Chain implememtations

Change in engine so the selected implementation is more abstract and has some kind of fall-back.

1. if intern or extern, try to get the specific implementation
2. else try to compile the body
3. else compile an error

The point is top prepare a future PR where intern and extern methods could provide an alternative (slow or degraded) implementation.
Voici venu le jour où notre compilateur n'a plus rien à envier au Texan moyen...

![image](https://cloud.githubusercontent.com/assets/583144/4190400/303f0bc4-3785-11e4-8577-7ce9eec10b10.png)

Je propose un message un peu plus explicatif...