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/Orders/UnitOrderGenerator.cs

@@ -37,7 +37,7 @@ namespace OpenRA.Orders
 			var orders = world.Selection.Actors
 				.Select(a => OrderForUnit(a, target, mi))
 				.Where(o => o != null)
-				.ToArray();
+				.ToList();
 
 			var actorsInvolved = orders.Select(o => o.Actor).Distinct();
 			if (actorsInvolved.Any())
@@ -81,8 +81,7 @@ namespace OpenRA.Orders
 
 			var orders = world.Selection.Actors
 				.Select(a => OrderForUnit(a, target, mi))
-				.Where(o => o != null)
-				.ToArray();
+				.Where(o => o != null);
 
 			var cursorName = orders.Select(o => o.Cursor).FirstOrDefault();
 			return cursorName ?? (useSelect ? "select" : "default");