- 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

[ticket:2777]

provided via the :func:`.event.remove` function.
[ticket:2268]
- reorganization of event.py module into a package; with the addition of the
docstring work as well as the new registry for removal, there's a lot more code now.
the package separates concerns and provides a top-level doc for each subsection
of functionality
- the remove feature works by providing the EventKey object which associates
the user-provided arguments to listen() with a global, weak-referencing registry.
This registry stores a collection of _ListenerCollection and _DispatchDescriptor
objects associated with each set of arguments, as well as the wrapped function
which was applied to that collection.  The EventKey can then be recreated for
a removal, all the _ListenerCollection and _DispatchDescriptor objects are located,
and the correct wrapped function is removed from each one.

:class:`.AttributeImpl` as an "initiator" token has been changed;
the object is now an event-specific object called :class:`.attributes.Event`.
Additionally, the attribute system no longer halts events based
on a matching "initiator" token; this logic has been moved to be
specific to ORM backref event handlers, which are the typical source
of the re-propagation of an attribute event onto subsequent append/set/remove
operations.  End user code which emulates the behavior of backrefs
must now ensure that recursive event propagation schemes are halted,
if the scheme does not use the backref handlers.   Using this new system,
backref handlers can now peform a
"two-hop" operation when an object is appended to a collection,
associated with a new many-to-one, de-associated with the previous
many-to-one, and then removed from a previous collection.   Before this
change, the last step of removal from the previous collection would
not occur.
[ticket:2789]

"propagate" flags could potentially be mis-configured in some
"unmapped base class" configurations.  Also in 0.8.3.
[ticket:2786]

with @_legacy_signature, will inspect incoming listener functions
to see if they match an older signature, will wrap into a newer sig
- add an event listen argument named=True, will send all args as
kw args so that event listeners can be written with **kw, any combination
of names
- add a doc system to events that writes out the various calling styles
for a given event, produces deprecation messages automatically.
a little concerned that it's a bit verbose but will look at it up
on RTD for awhile to get a feel.
- change the calling signature for bulk update/delete events - we have
the BulkUD object right there, and there's at least six or seven things
people might want to see, so just send the whole BulkUD in
[ticket:2775]