Change XMLWriter resources to objects

Change XMLWriter resources to objects

While we generally prefer objects over resources for quite a while, the
procedural XMLWriter API still uses resources, although there is
already an object-oriented API which uses objects.  This dichotomy
makes no sense, slightly complicates the implementation, and doesn't
allow a stepwise migration to the object-oriented API, which might be
desired.  Thus we completely drop the XMLWriter resources in favor of
XMLWriter objects.

We consider the minor BC break acceptable for a major version, since
only explicit type checks (`is_resource()`, `gettype()` etc.) need to
be adapted.