Link to state, not object, for mutable extension

Link to state, not object, for mutable extension

The ``sqlalchemy.ext.mutable`` extension now tracks the "parents"
collection using the :class:`.InstanceState` associated with objects,
rather than the object itself. The latter approach required that the object
be hashable so that it can be inside of a ``WeakKeyDictionary``, which goes
against the behavioral contract of the ORM overall which is that ORM mapped
objects do not need to provide any particular kind of ``__hash__()`` method
and that unhashable objects are supported.

Fixes: #6020
Change-Id: I414c8861bc5691f5f320dac3eb696f4b8c37d347