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/UtilityCommands/LegacyMapImporter.cs

@@ -187,8 +187,7 @@ namespace OpenRA.Mods.Common.UtilityCommands
 			var wps = file.GetSection("Waypoints")
 					.Where(kv => Exts.ParseIntegerInvariant(kv.Value) > 0)
 					.Select(kv => Pair.New(Exts.ParseIntegerInvariant(kv.Key),
-						LocationFromMapOffset(Exts.ParseIntegerInvariant(kv.Value), mapSize)))
-					.ToArray();
+						LocationFromMapOffset(Exts.ParseIntegerInvariant(kv.Value), mapSize)));
 
 			// Add waypoint actors
 			foreach (var kv in wps)