:class:`.DeferredReflection`
to define classes pending for reflection, if some subset of those
classes were discarded before the :meth:`.DeferredReflection.prepare`
method were called to reflect and map the class, a strong reference
to the class would remain held within the declarative internals.
This internal collection of "classes to map" now uses weak
references against the classes themselves.

automatic reflection support for the "secondary" table referred
to by a :func:`.relationship`.   "secondary", when specified
either as a string table name, or as a :class:`.Table` object with
only a name and :class:`.MetaData` object will also be included
in the reflection process when :meth:`.DeferredReflection.prepare`
is called. [ticket:2865]
- clsregistry._resolver() now uses a stateful _class_resolver()
class in order to handle the work of mapping strings to
objects.   This is to provide for simpler extensibility, namely
a ._resolvers collection of ad-hoc name resolution functions;
the DeferredReflection class adds its own resolver here in order
to handle relationship(secondary) names which generate new
Table objects.

- rework the event system so that event modules load after their
targets, dependencies are reversed
- create an improved strategy lookup system for the ORM
- rework the ORM to have very few import cycles
- move out "importlater" to just util.dependency
- other tricks to cross-populate modules in as clear a way as possible

that ``discriminator_on_association.py`` makes use of single table
inheritance do the work with the "discriminator".  Also
added a true "generic foreign key" example, which works similarly
to other popular frameworks in that it uses an open-ended integer
to point to any other table, foregoing traditional referential
integrity.  While we don't recommend this pattern, information wants
to be free.  Also in 0.8.3.

- Added a convenience class decorator :func:`.as_declarative`, is
a wrapper for :func:`.declarative_base` which allows an existing base
class to be applied using a nifty class-decorated approach.  Also
in 0.8.3.

where since it considers the current class' ``__table__``, was
sensitive to when it was called.  This is 0.7's behavior also,
but in 0.7 things tended to "work out" within events like
``__mapper_args__()``.  :func:`.has_inherited_table` now only
considers superclasses, so should return the same answer
regarding the current class no matter when it's called
(obviously assuming the state of the superclass).
[ticket:2656]