Checked LINQ queries and collections for inefficiencies.

- Made Array.IndexOf available via extension method.
- Made ToHashSet extension method.
- Change collections queried often via Contains into sets.
- Avoid Count() extension if Count or Length property exist.
- Made Count() > 0 checks and variations calls to Any() instead.
- Don't call ToList/ToArray if there is no benefit to materializing the sequence.
- If the sequence does benefit from materialization, follow this general pattern:
  - Collection queried often via Contains use ToHashSet to speed up lookups.
  - Short lived variables use ToList. This is because ToArray requires an extra copy to output the final size.
  - Collections persisted into fields or for a long time use ToArray to minimize memory overhead.

OpenRA.Game/Effects/FlashTarget.cs

@@ -25,8 +25,11 @@ namespace OpenRA.Effects
 			this.target = target;
 			player = asPlayer;
 			remainingTicks = ticks;
-			foreach (var e in target.World.Effects.OfType<FlashTarget>().Where(a => a.target == target).ToArray())
-				target.World.Remove(e);
+			target.World.RemoveAll(effect =>
+			{
+				var flashTarget = effect as FlashTarget;
+				return flashTarget != null && flashTarget.target == target;
+			});
 		}
 
 		public void Tick(World world)