Put new levels at the right position in the list

New levels need to go at the *end* of the list of levels, not the
beginning - otherwise we jump to the proper position on the new level
but the current level stays the same (oops).

src/Xanthous/App.hs

@@ -300,8 +300,7 @@ handleCommand GoDown = do
     let newLevelNum = Levels.pos levs + 1
     levs' <- nextLevel (levelToEntityMap <$> genLevel newLevelNum) levs
     cEID <- use characterEntityID
-    pCharacter <- use $ entities . at cEID
-    entities . at cEID .= Nothing
+    pCharacter <- entities . at cEID <<.= Nothing
     levels .= levs'
     entities . at cEID .= pCharacter
   else say_ ["cant", "goDown"]