Start replacing IntMap by EnumMap.Strict
Get rid of any IntMap and IntSet remaining
Make type Point abstract

It's now possible thanks to EnumMaps.
Disentangle LevelId and level depth
Get rid of most uses of Data.Map and Data.Set
Overhaul items, add ItemId, etc.
Use Data.Map.Strict instead of Data.Map everywhere

In some places it matters.
Make sure clients use UI one at a time
Pick better moments to display frames
Fix half of attack animation invisible
Fix killing actor of a non-human faction
Fix restore from a backup savefile