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/Group.cs

@@ -15,7 +15,7 @@ namespace OpenRA
 {
 	public class Group
 	{
-		List<Actor> actors;
+		Actor[] actors;
 		int id;
 
 		static int nextGroup;
@@ -24,7 +24,7 @@ namespace OpenRA
 
 		public Group(IEnumerable<Actor> actors)
 		{
-			this.actors = actors.ToList();
+			this.actors = actors.ToArray();
 
 			foreach (var a in actors)
 				a.Group = this;