Allow dialect-specific stringification

Allow dialect-specific stringification

Dialect-specific constructs such as
:meth:`_postgresql.Insert.on_conflict_do_update` can now stringify in-place
without the need to specify an explicit dialect object.  The constructs,
when called upon for ``str()``, ``print()``, etc. now have internal
direction to call upon their appropriate dialect rather than the
"default"dialect which doesn't know how to stringify these.   The approach
is also adapted to generic schema-level create/drop such as
:class:`_schema.AddConstraint`, which will adapt its stringify dialect to
one indicated by the element within it, such as the
:class:`_postgresql.ExcludeConstraint` object.

mostly towards being able to provide doctest-style
examples for "on conflict" constructs using print statements.

Change-Id: I4b855516fe6dee2df77744c1bb21a373d7fbab93