Add Executable to DefaultGenerator

Add Executable to DefaultGenerator

Fixed the class hierarchy for the :class:`_schema.Sequence` and the more
general :class:`_schema.DefaultGenerator` base, as these are "executable"
as statements they need to include :class:`_sql.Executable` in their
hierarchy, not just :class:`_roles.StatementRole` as was applied
arbitrarily to :class:`_schema.Sequence` previously. The fix allows
:class:`_schema.Sequence` to work in all ``.execute()`` methods including
with :meth:`_orm.Session.execute` which was not working in the case that a
``do_orm_execute()`` handler was also established.

Fixes: #6668
Change-Id: I0d192258c7cbd1bce2552f9e748e8fdd680dc45f