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.Mods.Common/Traits/World/SmudgeLayer.cs

@@ -70,7 +70,7 @@ namespace OpenRA.Mods.Common.Traits
 			}
 
 			// Add map smudges
-			foreach (var s in w.Map.Smudges.Value.Where(s => smudges.Keys.Contains(s.Type)))
+			foreach (var s in w.Map.Smudges.Value.Where(s => smudges.ContainsKey(s.Type)))
 			{
 				var smudge = new Smudge
 				{