: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.

attribute on declarative to simply refer directly to an :class:`.InstrumentedAttribute`
on a superclass or on the class itself, and it
acts more or less like a synonym; in 0.9, this fails to set up enough
bookkeeping to keep up with the more liberalized backref logic
from :ticket:`2789`.  Even though this use case was never directly
considered, it is now detected by declarative at the "setattr()" level
as well as when setting up a subclass, and the mirrored/renamed attribute
is now set up as a :func:`.synonym` instead. [ticket:2900]

:class:`.Column` is mapped multiple times under different properties
(which typically should be a :func:`.synonym` instead) or if two
or more :class:`.Column` objects are given the same name, raising
a warning if this condition is detected. [ticket:2828]

doesn't resolve to a class or mapper has been corrected to work
the same way as when a non-string arg is received, which indicates
the name of the relationship which had the configurational error.
[ticket:2888]

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.

as the string name of a class or other argument within
declarative using :func:`.relationship`.

correctly with table or column names that contain non-ASCII
characters. [ticket:2869]

around for a long time
- association proxy now returns None for proxied scalar that is also None, rather
than raising AttributeError. [ticket:2810]

"serializer" can just be ditched, or if somehow these deterministic issues
can be fixed (everyone implements __reduce__ maybe?)