the creation of strong references within the Session;
an object will no longer have an internal reference cycle
created if it's in the transient state or moves into the
detached state - the strong ref is created only when the
object is attached to a Session and is removed when the
object is detached.  This makes it somewhat safer for an
object to have a `__del__()` method, even though this is
not recommended, as relationships with backrefs produce
cycles too.  A warning has been added when a class with
a `__del__()` method is mapped.
[ticket:2708]

associated with the ORM or its extensions can be retrieved.
This fulfills the common request of being able to inspect
all :class:`.QueryableAttribute` descriptors in addition to
extension types such as :class:`.hybrid_property` and
:class:`.AssociationProxy`.  See :attr:`.Mapper.all_orm_descriptors`.

    mapped classes allows that unmapped classes
    can be specified for instance- and mapper-events.
    The established events will be automatically
    set up on subclasses of that class when the
    propagate=True flag is passed, and the
    events will be set up for that class itself
    if and when it is ultimately mapped.
    [ticket:2585]

  - [bug] The instrumentation events class_instrument(),
    class_uninstrument(), and attribute_instrument()
    will now fire off only for descendant classes
    of the class assigned to listen().  Previously,
    an event listener would be assigned to listen
    for all classes in all cases regardless of the
    "target" argument passed.  [ticket:2590]

that change object state as these aren't intended for public
use.

and the entire related system of alternate
class implementation is now moved out
to sqlalchemy.ext.instrumentation.   This is
a seldom used system that adds significant
complexity and overhead to the mechanics of
class instrumentation.  The new architecture
allows it to remain unused until
InstrumentationManager is actually imported,
at which point it is bootstrapped into
the core.

- break out key mechanics of loading objects
into new "orm.loading" module, removing implementation
details from both mapper.py and query.py. is analogous
to persistence.py
- some other cleanup and old cruft removal

ORM, including the MutableType class as well
as the mutable=True flag on PickleType
and postgresql.ARRAY has been removed.
In-place mutations are detected by the ORM
using the sqlalchemy.ext.mutable extension,
introduced in 0.7.   The removal of MutableType
and associated constructs removes a great
deal of complexity from SQLAlchemy's internals.
The approach performed poorly as it would incur
a scan of the full contents of the Session
when in use. [ticket:2442]