- the original rationale for defaulting the user-defined namespace

- the original rationale for defaulting the user-defined namespace
to "sa." was to force users to deal with making sure their custom
types came from a fixed module somewhere.  However, it's not worth
defending this rationale.

The default value of the
:paramref:`.EnvironmentContext.configure.user_module_prefix`
parameter is **no longer the same as the SQLAlchemy prefix**.
When omitted, user-defined types will now use the ``__module__``
attribute of the type class itself when rendering in an
autogenerated module.

fixes #229