- 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.

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.

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]

make sure that the DBAPI connection passed to the reset-on-return events/dialect hooks
is also a "fairy", so that dictionaries like "info" are available.  [ticket:2770]
- rework the execution_options system so that the dialect is given the job of making
any immediate adjustments based on a set event.  move the "isolation level" logic to use
this new system.   Also work things out so that even engine-level execution options
can be used for things like isolation level; the dialect attaches a connect-event
handler in this case to handle the task.
- to support this new system as well as further extensibiltiy of execution options
add events engine_connect(), set_connection_execution_options(), set_engine_execution_options()

whereas the other do_rollback_twophase(), savepoint etc. work with
:class:`.Connection`.  the context on these are different as twophase/savepoint
are available at the :class:`.Connection` level, whereas commit/rollback are needed
at a lower level as well.  Rename the argument to "dbapi_connection" when the conneciton
is in fact the DBAPI interface.
- start thinking about being able to track "autocommit" vs. "commit", but not sure
we have a need for this yet.
- have Pool call out to a Dialect for all rollback/commit/close operations now.  Pool
no longer calls DBAPI methods directly.  May use this for a workaround for [ticket:2611]
- add a new Pool event reset() to allow the pool's reset of the connection to be intercepted.
- remove methods in Informix dialect which appear to be hardcoding some isolation
settings on new Transaction only; the isolation API should be implemented for Informix.
also removed "flag" for transaction commit/rollback being not available; this should
be based on server/DBAPI version and we will need someone with test access in order
to help determine how this should work

    fires off for so-called "_cursor_execute"
    events, which are usually special-case
   executions of primary-key bound sequences
    and default-generation SQL
    phrases that invoke separately when RETURNING
    is not used with INSERT.  [ticket:2459]

now be associated with individual
Connection objects, not just Engine
objects.  [ticket:2511]