DATACMNS-590 - Fixed calculation of nested generics in ParentTypeAwareTypeInformation.

So far, the lookup of the type variable map was preferring the type variable maps detected on parent types in nested structures. This caused the concrete type variables in nested types not being considered correctly which caused the type resolution to fall back to the generic bounds.

We now accumulate the type variable maps to avoid having to lookup a certain map in the nesting hierarchy. The core fix is in ParentTypeAwareTypeInformation's constructor and mergeMaps(…) respectively. Simplified the handling of type variable maps and made proper use of generics throughout the class hierarchy.