Isolate world model on a global actor

[fwcd]

This branch introduces the global @WorldActor isolating accesses to the world model throughout the application, thereby leveraging Swift 5.5's new concurrency and async/await (and hopefully eliminating potential data races).

Some challenges with this approach:

• Performance is noticeably worse, especially as the player moves through the world due to all player state, including the position, being stored in the world model
  • A potential solution would be to separate the world model from e.g. player state, which could be moved onto another actor

See also:

• Investigate threading and concurrency issues #26