API changes

* removed StackedWriter (no need currently for switching writers during template compilation)
* expanded ObjectModel in order to remove o.a.s.s.s.impl.common.Dynamic
* refactored RenderContext to remove references to Dynamic (the ObjectModel service can be used instead)
* moved BaseRenderUnit to impl (no need to export it)

TODOs:

* remove UnitLocator
* expose RenderContext as an interface