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/Map/MapCache.cs

@@ -79,7 +79,7 @@ namespace OpenRA
 			foreach (var p in maps.Values)
 				p.UpdateRemoteSearch(MapStatus.Searching, null);
 
-			var url = Game.Settings.Game.MapRepository + "hash/" + string.Join(",", maps.Keys.ToArray()) + "/yaml";
+			var url = Game.Settings.Game.MapRepository + "hash/" + string.Join(",", maps.Keys) + "/yaml";
 
 			Action<DownloadDataCompletedEventArgs, bool> onInfoComplete = (i, cancelled) =>
 			{