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/Widgets/PerfGraphWidget.cs

@@ -26,7 +26,7 @@ namespace OpenRA.Widgets
 			Game.Renderer.LineRenderer.DrawLine(origin + new float2(100, 0) * basis, origin + new float2(100, 100) * basis, Color.White, Color.White);
 
 			var k = 0;
-			foreach (var item in PerfHistory.Items.Values.ToArray())
+			foreach (var item in PerfHistory.Items.Values)
 			{
 				var n = 0;
 				item.Samples().Aggregate((a, b) =>
@@ -55,7 +55,7 @@ namespace OpenRA.Widgets
 			}
 
 			k = 0;
-			foreach (var item in PerfHistory.Items.Values.ToArray())
+			foreach (var item in PerfHistory.Items.Values)
 			{
 				Game.Renderer.Fonts["Tiny"].DrawText(item.Name, new float2(rect.Left, rect.Top) + new float2(18, 10 * k - 3), Color.White);
 				++k;