Remove some stuff that I expect to rewrite
Caching and optimization
Memoize the copy method of Character
Memoize a little better
Even better memoization.

I think I did cache invalidation correctly here
Remove the copy-caching in Character because it was buggy, instead focus on making the caching good for the entities inside Character
Generally do better at delegating to avatars
copy Character *before* calling the function, now
make arrival_time and next_arrival_time use the cache.

The way I did this may cause trouble with time travel if it's done between two distant ticks of the same branch, when the ticks in between haven't been cached yet.
Changes to CharacterImage that make it easier to cache, and new Character.copy() that takes advantage
Merge branch 'master' of github.com:LogicalDash/LiSE