CellIsEvaluating perf optimization

If an actor has Mobile, it implements IOccupySpace
so we can use OccupiesSpace to save a trait look-up.

OpenRA.Mods.Common/Activities/Move/Move.cs

@@ -341,7 +341,7 @@ namespace OpenRA.Mods.Common.Activities
 		{
 			foreach (var actor in self.World.ActorMap.GetActorsAt(cell))
 			{
-				var move = actor.TraitOrDefault<Mobile>();
+				var move = actor.OccupiesSpace as Mobile;
 				if (move == null || !move.IsTraitEnabled() || !move.IsLeaving())
 					return false;
 			}