gst/: Use _CAST macros to avoid unneeded type checking.

gst/: Use _CAST macros to avoid unneeded type checking.

Original commit message from CVS:
* gst/gst.c: (gst_debug_help):
* gst/gstplugin.c: (gst_plugin_finalize), (gst_plugin_list_free):
* gst/gstpluginfeature.c: (gst_plugin_feature_finalize),
(gst_plugin_feature_list_free):
* gst/gstregistry.c: (gst_registry_add_plugin),
(gst_registry_add_feature), (gst_registry_plugin_filter),
(gst_registry_feature_filter), (gst_registry_find_plugin),
(gst_registry_find_feature), (gst_registry_get_plugin_list),
(gst_registry_lookup_feature_locked), (gst_registry_lookup_locked):
* gst/gstregistryxml.c: (load_feature),
(gst_registry_xml_read_cache), (gst_registry_xml_write_cache):
* gst/gstminiobject.c: (gst_mini_object_unref),
(gst_mini_object_replace), (gst_value_mini_object_free),
(gst_value_mini_object_copy):
Use _CAST macros to avoid unneeded type checking.
Added some more G_UNLIKELY.