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/Server/Server.cs

@@ -406,7 +406,7 @@ namespace OpenRA.Server
 		public void DispatchOrdersToClients(Connection conn, int frame, byte[] data)
 		{
 			var from = conn != null ? conn.PlayerIndex : 0;
-			foreach (var c in Conns.Except(conn).ToArray())
+			foreach (var c in Conns.Except(conn).ToList())
 				DispatchOrdersToClient(c, from, frame, data);
 		}
 
@@ -517,9 +517,7 @@ namespace OpenRA.Server
 
 		public void DropClient(Connection toDrop, int frame)
 		{
-			if (PreConns.Contains(toDrop))
-				PreConns.Remove(toDrop);
-			else
+			if (!PreConns.Remove(toDrop))
 			{
 				Conns.Remove(toDrop);