Wed Mar  7 19:02:51 2001  Tim Janik  <timj@gtk.org>

        * gtype.c (type_node_add_iface_entry_W): catch when adding an interface
        to an ancestor of a child that already conforms to this interface.
        currently we spew a warning here, should we be silent?
        (g_type_interface_add_prerequisite): new function to add a prerequisite
        type to an interface, that must succeed an instance is_a prerequisite
        type check before the interface can be added to an instance. the
        prerequisite types are also suuported in is_a checks of the interface.
        (g_type_instance_is_a):
        (g_type_check_instance_cast):
        (g_type_check_instance): cleanups and optimizations.
        (g_type_class_is_a):
        (g_type_check_class_cast): same, also prevented these from accepting
        interface types, as class structures don't nest interfaces.