NEVER_SET symbol could easily leak into a lazyload query, subsequent
to the flush of a pending object.  This would occur typically
for a many-to-one relationship that does not use a simple
"get" strategy.   The good news is that the fix improves efficiency
vs. 0.9, because we can now skip the SELECT statement entirely
when we detect NEVER_SET symbols present in the parameters; prior to
:ticket:`3061`, we couldn't discern if the None here were set or not.
fixes #3368

between fully and match on those, rather than trying to
compare selectables; fixes #3347

for memoization on a class that uses slots.
- apply many more __slots__.  mem use for nova now at 46% savings

improved in conjunction with loader directives such as
:func:`.joinedload` and :func:`.contains_eager` such that if
two :meth:`.PropComparator.of_type` modifiers of the same
base type/path are encountered, they will be joined together
into a single "polymorphic" entity, rather than replacing
the entity of type A with the one of type B.  E.g.
a joinedload of ``A.b.of_type(BSub1)->BSub1.c`` combined with
joinedload of ``A.b.of_type(BSub2)->BSub2.c`` will create a
single joinedload of ``A.b.of_type((BSub1, BSub2)) -> BSub1.c, BSub2.c``,
without the need for the ``with_polymorphic`` to be explicit
in the query.
fixes #3256

:meth:`.Query.outerjoin`, or the standalone :func:`.orm.join` /
:func:`.orm.outerjoin` functions to a single-inheritance subclass will
now include the "single table criteria" in the ON clause even
if the ON clause is otherwise hand-rolled; it is now added to the
criteria using AND, the same way as if joining to a single-table
target using relationship or similar.
fixes #3222

- prop.compare() isn't needed; replace with prop._with_parent()
for relationships
- update docs in orm/interfaces