Implement swap using evaluators.

Eigen/src/Core/Swap.h

@@ -119,6 +119,8 @@ template<typename ExpressionType> class SwapWrapper
       _other.template writePacket<LoadMode>(index, tmp);
     }
 
+    ExpressionType& expression() const { return m_expression; }
+
   protected:
     ExpressionType& m_expression;
 };