Add SelectBase.exists() method as it seems strange this is
not available already.  The Exists construct itself does
not provide full SELECT-building capabilities so it makes
sense this should be used more like a scalar_subquery.

Make sure stream_results is getting set up when yield_per
is used, for 2.0 style statements as well.  this was
hardcoded inside of Query.yield_per() and is now moved
to take place within QueryContext.

Change-Id: Icafcd4fd9b708772343d56edf40995c9e8f835d6

The :class:`_schema.Table` class now raises a deprecation warning
when columns with the same name are defined. To replace a column a new
parameter :paramref:`_schema.Table.append_column.replace_existing` was
added to the :meth:`_schema.Table.append_column` method.

The :meth:`_expression.ColumnCollection.contains_column` will now
raises an error when called with a string, suggesting the caller
to use ``in`` instead.

Co-authored-by: Federico Caselli <cfederico87@gmail.com>
Change-Id: I1d58c8ebe081079cb669e7ead60886ffc1b1a7f5

Reworked the proxy creation used by scoped_session() to be
based on fully copied code with augmented docstrings and
moved it into langhelpers.  asyncio session, engine,
connection can now take
advantage of it so that all non-async methods are availble.

Overall implementation of most important accessors / methods
on AsyncConnection, etc. , including awaitable versions
of invalidate, execution_options, etc.

In order to support an event dispatcher on the async
classes while still allowing them to hold __slots__,
make some adjustments to the event system to allow
that to be present, at least rudimentally.

Fixes: #5628
Change-Id: I5eb6929fc1e4fdac99e4b767dcfd49672d56e2b2

Fixes: #5634
Change-Id: Ie8d4076ee35234b535a04e6fb9321096df3f648b

Repaired a function-level import that was not using SQLAlchemy's standard
late-import system within the sqlalchemy.exc module.

Moved preloaded to sqlalchemy.util.preloaded so that it
does not depend on langhelpers which depends on exc.

Fixes: #5632
Change-Id: I61b7ce9cd461071ce543714739f67aa5aeb47fd6

Fixes: #5223
Change-Id: I0952e54ed9af2952ea340be1945311376ffc1ad2

It's better, the majority of these changes look more readable to me.
also found some docstrings that had formatting / quoting issues.

Change-Id: I582a45fde3a5648b2f36bab96bad56881321899b

The automatic uniquing of rows on the client side is turned off for the new
:term:`2.0 style` of ORM querying.  This improves both clarity and
performance.  However, uniquing of rows on the client side is generally
necessary when using joined eager loading for collections, as there
will be duplicates of the primary entity for each element in the
collection because a join was used.  This uniquing must now be manually
enabled and can be achieved using the new
:meth:`_engine.Result.unique` modifier.   To avoid silent failure, the ORM
explicitly requires the method be called when the result of an ORM
query in 2.0 style makes use of joined load collections.    The newer
:func:`_orm.selectinload` strategy is likely preferable for eager loading
of collections in any case.

This changeset also fixes an issue where ORM-style "single entity"
results would not apply unique() correctly if results were returned
as tuples.

Fixes: #4395
Change-Id: Ie62e0cb68ef2a6d2120e968b79575a70d057212e