* Makefile.in (COMMON_OBS): Add registry.o.
* registry.c: New file.
* registry.h (struct registry_container): Declare.
(registry_data_callback): New typedef.
(struct registry_data, struct registry_data_registration, struct
registry_data_registry): New type.
(register_data_with_cleanup, registry_alloc_data)
(registry_callback_adaptor, registry_clear_data)
(registry_container_free_data, registry_set_data, registry_data):
Declare.
(DEFINE_REGISTRY): Refactor structures and functions as shims over
the new common structures and functions.
(DECLARE_REGISTRY): Declare struct TAG ## _data. Use the tagged
callback typedefs.
