now be constructed such that the visit methods receive an indication
of the owning expression object, if any.  Any visit method that
accepts keyword arguments (e.g. ``**kw``) will in most cases
receive a keyword argument ``type_expression``, referring to the
expression object that the type is contained within.  For columns
in DDL, the dialect's compiler class may need to alter its
``get_column_specification()`` method to support this as well.
The ``UserDefinedType.get_col_spec()`` method will also receive
``type_expression`` if it provides ``**kw`` in its argument
signature.
fixes #3074

repaired to work more usefully with tables that have Python-
side default values and/or functions, as well as server-side
defaults. The feature will now work with a dialect that uses
"positional" parameters; a Python callable will also be
invoked individually for each row just as is the case with an
"executemany" style invocation; a server- side default column
will no longer implicitly receive the value explicitly
specified for the first row, instead refusing to invoke
without an explicit value. fixes #3288

Needed for alembic autogenerate rendering.

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

get PG stuff working

size of the many per-column objects we're hitting, but somehow the overall memory is
hardly being reduced at all in initial testing

- test_schema_2 is only on PG and doesn't need a drop all, omit this for now
- py3k has exception.args[0], not message