v0.21.0 bookmarks

• Added the bookmark mapping/method. This is a convenient way to store points
  in time and travel to them. You can call it with whatever name you want to
  give your bookmark to make a new one, then call it with that name again
  to travel back to that time. Bookmarks are not versioned. If you want
  a point in time that changes in response to time travel, store the time in
  the engine's universal mapping.
• Added support for storing the value None. Values deleted from dictionaries
  are now represented by the ellipsis object.
• Renamed the user property of nodes to leader.
• Removed what remained of multigraphs, which may never be supported. This
  changed the database schema, so old playthroughs are incompatible.
• Generally decrufted the codebase.