Add PN NPN awareness into the backends.

This MR aims to give the Backends the capability to assume certain things about the executing enviroment. E.g. If the regions are properly nested or not.

Therefore, we introduce two different kinds of AnnotationStrategies. This MR introduces the concepts, and in the following PR @vseitz will add some more flesh to skeleton, providing the current hirarchie of the Stack to the ProperlyNested backends.