add ToString() for Pair, to make debug saner.

OpenRA.FileFormats/Primitives/Pair.cs

@@ -56,6 +56,11 @@ namespace OpenRA.FileFormats
 		public static U AsSecond(Pair<T, U> p) { return p.Second; }
 
         public Pair<U, T> Swap() { return Pair.New(Second, First); }
+
+		public override string ToString()
+		{
+			return "({0},{1})".F(First, Second);
+		}
 	}
 
     public static class Pair