- moved class-level attributes placed by the attributes package into a _class_state

- moved class-level attributes placed by the attributes package into a _class_state
variable attached to the class.
- mappers track themselves primarily using the "mappers" collection on _class_state.
ClassKey is gone and mapper lookup uses regular dict keyed to entity_name; removes
a fair degree of WeakKeyDictionary overhead as well as ClassKey overhead.
- mapper_registry renamed to _mapper_registry; is only consulted by the
compile_mappers(), mapper.compile() and clear_mappers() functions/methods.