nsf.c - base MixinInvalidateObjOrders() on DependentSubClasses() and avoid the need of using a separate hash table for class-mixin handling. The new implementation is several times faster and improves the speed of the functions CleanupDestroyClass(), SuperclassAdd() and NsfRelationClassMixinsSet(). Adding a class-mixin to ::xotcl::Object in OpenACS is more than 4x faster. - remove obsolete function MixinResetOrderForInstances() - rename ResetOrderOfClassesUsedAsMixins() to ResetOrderOfObjectsUsingThisClassAsObjectMixin() - used consistenlty DependentSubClasses() instead of TransitiveSubClasses() for invalidations. - extend regression test