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
- white-space change