tagged_items updates

tagged_items updates

- Expose tagged_item functions in new header tagged_items.h
- Make object_id functions inline functions in object_id.h
- Make inode_get_tagged_item() return stored length, not aligned length
- Add a new function inode_set_tagged_data() which removes existing
  items before setting the new one, and use it for inode_set_object_id()
- Make inode_add_tagged_item() append item rather than prepend
- Keep items 8-byte aligned in memory