Add Registry to replace set_*_class()

This new class handles creating objects and calling static methods.

Objects created by this class can opt to receive a reference to it via a `set_registry` method, which takes a `SimplePie_Registry` reference as its only parameter.

Classes registered via the old Core::set_*_class() methods are marked as legacy, and parameters may be shuffled to compensate. All new classes should use `$feed->get_registry()->register($type, $class)` instead.