Remove or scale-down Forest class

[aionick]

With the new ClassHierarchy the previous one (ClassHierarchyForest) likely can be heavily reduced in terms of functionality. The previous forest is currently only be used to compute the sizes of the classes and so it should just become the most mininal data structure we need in order to accomplish this task - the forest aspect is likely overkill now. If the forest aspect is required to compute these sizes then it probably makes more sense to just move this capacity into the new hierarchy and entirely remove the old one.