Use an XmlParser object instead of a resource. This is an internal
representation change, not a conversion to OO APIs. XmlParser objects
cannot be explicitly constructed, they are created through the usual
xml_parser_* APIs.
This change allows us to provide a proper get_gc() implementation,
thus resolving bugs #72793 and #76874.
xml_parser_free() is a no-op now and need not be called anymore.