Add solution for 2016 Day 22

src/common/lib/Array2D.h

@@ -500,6 +500,11 @@ struct Array2D
     return ConstIterator{*this, Index2D{0, height}};
   }
 
+  bool operator<(const Array2D<T>& other) const
+  {
+    return array2D < other.array2D;
+  }
+
   friend std::ostream& operator<<(std::ostream& os, const Array2D& array2D)
   {
     for (int y = 0; y < array2D.height; y++)