objects, such that they were prevented from being called outside
of the declarative process; this is related to the enhancements
of #3150 which allow ``@declared_attr`` to return a value that is
cached based on the current class as it's being configured.
The exception raise has been removed, and the behavior changed
so that outside of the declarative process, the function decorated by
``@declared_attr`` is called every time just like a regular
``@property``, without using any caching, as none is available
at this stage.
fixes #3331

simple but unusual system allows for a dramatic savings in Python
overhead for the construction and processing of orm :class:`.Query`
objects, from query construction up through rendering of a string
SQL statement.
fixes #3054

of a declarative inheritance hierarchy would prevent attributes
and configuration being correctly propagated from the base class
to the inheriting class.
fixes #3219 fixes #3240

subclass, the value returned by the descriptor is not available
because the superclass is already mapped with the InstrumentedAttribute,
until the subclass is mapped.  We add a setattr() to set up that
attribute so that the __mapper_args__ hook and possibly others
have access to the "cascaded" version of the attribute within
the call.

on engines

when using :class:`.AbstractConcreteBase` in conjunction with
a subclass that declares ``__abstract__``.
fixes #3185

that are now affixed to _MapperConfig
- declarative now creates column copies ahead of time
so that they are ready to go for a declared_attr
- overhaul of declared_attr; memoization, cascading modifier
- A relationship set up with :class:`.declared_attr` on
a :class:`.AbstractConcreteBase` base class will now be configured
on the abstract base mapping automatically, in addition to being
set up on descendant concrete classes as usual.
fixes #2670
- The :class:`.declared_attr` construct has newly improved
behaviors and features in conjunction with declarative.  The
decorated function will now have access to the final column
copies present on the local mixin when invoked, and will also
be invoked exactly once for each mapped class, the returned result
being memoized.   A new modifier :attr:`.declared_attr.cascading`
is added as well. fixes #3150
- the original plan for #3150 has been scaled back; by copying
mixin columns up front and memoizing, we don't actually need
the "map properties later" thing.
- full docs + migration notes