[WIP] Introduce symfony service container and testwork extensions

Starts to replace the custom container with the Symfony one. Its wrapped inside it at the moment with a view to just having the symfony container in a later major version. As a result of trying to maintain b/c its quite hacky at the moment and would require quite a bit of clean up at a later stage.

Supports both current phpspec extensions and testwork extensions. Started moving some of the core services to testwork extensions. 
this won't work properly if an old extension is registered before a Testwork one, right ?