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.Mods.Common/Orders/PlaceBuildingOrderGenerator.cs

@@ -47,8 +47,8 @@ namespace OpenRA.Mods.Common.Orders
 			if (mi.Button == MouseButton.Right)
 				world.CancelInputMode();
 
-			var ret = InnerOrder(world, xy, mi).ToList();
-			if (ret.Count > 0)
+			var ret = InnerOrder(world, xy, mi).ToArray();
+			if (ret.Length > 0)
 				world.CancelInputMode();
 
 			return ret;