it to prevent excess memory usage with yield_per

objects that made use of the ``__clause_element__()`` method and
returned an object that was an ORM-mapped
:class:`.InstrumentedAttribute` and not explicitly a
:class:`.ColumnElement` would fail to be correctly
handled when passed as an expression to :meth:`.Session.query`.
The logic in 0.9 happened to succeed on this, so this use case is now
supported. fixes #3448

as failing with the new 'entity' key value added to
:attr:`.Query.column_descriptions`, the logic to discover the "from"
clause is again reworked to accommodate columns from aliased classes,
as well as to report the correct value for the "aliased" flag in these
cases.
fixes #3409

of ``entity`` to the :attr:`.Query.column_descriptions` accessor
would fail if the target entity was produced from a core selectable
such as a :class:`.Table` or :class:`.CTE` object.
fixes #3403  references #3320

changes made to the :class:`.Query` object's collection of entities
to load within the event would render in the SQL, but would not
be reflected during the loading process.
fixes #3387

same target more than once; it implicitly dedupes only in the case of
a relationship join, and due to :ticket:`3233`, in 1.0 a join
to the same table twice behaves differently than 0.9 in that it no
longer erroneously aliases.   To help document this change,
the verbiage regarding :ticket:`3233` in the migration notes has
been generalized, and a warning has been added when :meth:`.Query.join`
is called against the same target relationship more than once.
fixes #3367

FROM clauses when using the :meth:`.Query.update` or
:meth:`.Query.delete` methods; instead of silently ignoring these
fields if methods like :meth:`.Query.join` or
:meth:`.Query.select_from` has been called, an error is raised.
In 0.9.10 this only emits a warning.
fixes #3349
- don't needlessly call _compile_context() and build up a
whole statement that we never need.  Construct QueryContext
as it's part of the event contract, but don't actually call upon
mapper attributes; use more direct systems of determining the
update or delete table.
- don't realy need _no_select_modifiers anymore

This is useful to be able to pass in mysql_limit=1 from using the
ORM.