- 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

this is a dictionary where applications can store arbitrary
data local to a :class:`.Session`.
The contents of :attr:`.Session.info` can be also be initialized
using the ``info`` argument of :class:`.Session` or
:class:`.sessionmaker`.

as possible

of :meth:`.Session.begin_nested` would fail to correctly
roll back the transaction when a flush error occurred, instead
raising its own exception while leaving the session still
pending a rollback.  [ticket:2718]

- went through examples/ and cleaned out excess list() calls

the creation of strong references within the Session;
an object will no longer have an internal reference cycle
created if it's in the transient state or moves into the
detached state - the strong ref is created only when the
object is attached to a Session and is removed when the
object is detached.  This makes it somewhat safer for an
object to have a `__del__()` method, even though this is
not recommended, as relationships with backrefs produce
cycles too.  A warning has been added when a class with
a `__del__()` method is mapped.
[ticket:2708]