- 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]

    internally instead of "del" to avoid KeyError/warning
    during a non-determinate gc teardown [ticket:2267]

    circumstances could occur with the identity
    map, part of [ticket:2148]

    rare weakref callbacks during iterations.
    The mutex has been removed as it apparently
    can cause a reentrant (i.e. in one thread) deadlock,
    perhaps when gc collects objects at the point of
    iteration in order to gain more memory.  It is hoped
    that "dictionary changed during iteration" will
    be exceedingly rare as iteration methods internally
    acquire the full list of objects in a single values()
    call. [ticket:2087]