- 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

the import structure of many core modules.
``sqlalchemy.schema`` and ``sqlalchemy.types``
remain in the top-level package, but are now just lists of names
that pull from within ``sqlalchemy.sql``.  Their implementations
are now broken out among ``sqlalchemy.sql.type_api``, ``sqlalchemy.sql.sqltypes``,
``sqlalchemy.sql.schema`` and ``sqlalchemy.sql.ddl``, the last of which was
moved from ``sqlalchemy.engine``.  ``sqlalchemy.sql.expression`` is also
a namespace now which pulls implementations mostly from ``sqlalchemy.sql.elements``,
``sqlalchemy.sql.selectable``, and ``sqlalchemy.sql.dml``.
Most of the "factory" functions
used to create SQL expression objects have been moved to classmethods
or constructors, which are exposed in ``sqlalchemy.sql.expression``
using a programmatic system.  Care has been taken such that all the
original import namespaces remain intact and there should be no impact
on any existing applications.   The rationale here was to break out these
very large modules into smaller ones, provide more manageable lists
of function names, to greatly reduce "import cycles" and clarify the
up-front importing of names, and to remove the need for redundant
functions and documentation throughout the expression package.

we are pretty much back to the beginning, nothing to see here

since we are dealing with cycles in any case.

[ticket:2725]

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

if the given object was the subject of a :meth:`.Session.delete`
operation.
- An object that's deleted from a session will be de-associated with
that session fully after the transaction is committed, that is
the :func:`.object_session` function will return None.
[ticket:2658]

an "orphan" has been modified to more closely match the
behavior as that of persistent objects, which is that the object
is expunged from the :class:`.Session` as soon as it is
de-associated from any of its orphan-enabled parents.  Previously,
the pending object would be expunged only if de-associated
from all of its orphan-enabled parents.  The new flag ``legacy_is_orphan``
is added to :func:`.orm.mapper` which re-establishes the
legacy behavior. [ticket:2655]