through all possible set values

assume that the empty string, or a set with a single empty string
value, is in fact a set with a single empty string; instead, this
is by default treated as the empty set.  In order to handle persistence
of a :class:`.mysql.SET` that actually wants to include the blank
value ``''`` as a legitimate value, a new bitwise operational mode
is added which is enabled by the
:paramref:`.mysql.SET.retrieve_as_bitwise` flag, which will persist
and retrieve values unambiguously using their bitflag positioning.
Storage and retrieval of unicode values for driver configurations
that aren't converting unicode natively is also repaired.
fixes #3283

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

ORM functions that are derived as "public factory" symbols, which
should assist with documentation tools being able to report on the
target module.
fixes #3218

that are now affixed to _MapperConfig
- declarative now creates column copies ahead of time
so that they are ready to go for a declared_attr
- overhaul of declared_attr; memoization, cascading modifier
- A relationship set up with :class:`.declared_attr` on
a :class:`.AbstractConcreteBase` base class will now be configured
on the abstract base mapping automatically, in addition to being
set up on descendant concrete classes as usual.
fixes #2670
- The :class:`.declared_attr` construct has newly improved
behaviors and features in conjunction with declarative.  The
decorated function will now have access to the final column
copies present on the local mixin when invoked, and will also
be invoked exactly once for each mapped class, the returned result
being memoized.   A new modifier :attr:`.declared_attr.cascading`
is added as well. fixes #3150
- the original plan for #3150 has been scaled back; by copying
mixin columns up front and memoizing, we don't actually need
the "map properties later" thing.
- full docs + migration notes

constructs are now importable from the "from sqlalchemy" namespace,
just like every other Core construct.
- The implicit conversion of strings to :func:`.text` constructs
when passed to most builder methods of :func:`.select` as
well as :class:`.Query` now emits a warning with just the
plain string sent.   The textual conversion still proceeds normally,
however.  The only method that accepts a string without a warning
are the "label reference" methods like order_by(), group_by();
these functions will now at compile time attempt to resolve a single
string argument to a column or label expression present in the
selectable; if none is located, the expression still renders, but
you get the warning again. The rationale here is that the implicit
conversion from string to text is more unexpected than not these days,
and it is better that the user send more direction to the Core / ORM
when passing a raw string as to what direction should be taken.
Core/ORM tutorials have been updated to go more in depth as to how text
is handled.
fixes #2992