We can rid ourselves of a layer of indirection by subclassing
PublicInbox::Smsg instead of using a container object to hold
each $smsg. Furthermore, the `{id}' vs. `{mid}' field name
confusion is eliminated.
This reduces the size of the $rootset passed to walk_thread by
around 15%, that is over 50K memory when rendering a /$INBOX/
landing page.