fix(xanthous): Wipe memoized vision when the game updates

If the game steps forward, entities could be moving around, so we need
to wipe the memoized character vision.

Change-Id: If6ea6a8c4bed7aefef90a0a2ab17eff0af9b9bbe
Reviewed-on: https://cl.tvl.fyi/c/depot/+/3215
Reviewed-by: grfn <grfn@gws.fyi>
Tested-by: BuildkiteCI

users/grfn/xanthous/src/Xanthous/App/Time.hs

@@ -16,6 +16,7 @@ import           Xanthous.Game.State
 import           Xanthous.Game.Prompt
 import           Xanthous.Game.Lenses
 import           Control.Monad.State (modify)
+import qualified Xanthous.Game.Memo as Memo
 --------------------------------------------------------------------------------
 
 
@@ -26,6 +27,7 @@ stepGameBy ticks = do
     pEntity' <- step ticks pEntity
     entities . ix eid .= pEntity'
 
+  clearMemo Memo.characterVisiblePositions
   modify updateCharacterVision
 
   whenM (uses character isDead)