- pared down private and semi-private functions in the attributes package.

- pared down private and semi-private functions in the attributes package.
- simplified the process of establishment and unestablishment of
class management from a mapper perspective; class manager setup/teardown
is now symmetric (ClassManager would never be fully de-associated previously).
- class manager now unconditionally decorates __init__.  this has a slight
behavior change for an unmapped subclass of a mapped superclass, in that
InstanceState creation corresponds to that of the superclass.  This
still doesn't allow unmapped subclasses to be usable in mapper
situations, though.