Implement interface for NoAlias assignments.

* Rename the old copy_using_evaluators to noalias_copy_using_evaluators.
* Write a new copy_using_evaluators which strips NoAlias expression, if present,
  and calls noalias_copy_using_evaluators; in future, it will also take care of
  aliasing in products.
* Add expression() getter to NoAlias.

Eigen/src/Core/NoAlias.h

@@ -97,6 +97,11 @@ class NoAlias
     { return m_expression.derived() -= CoeffBasedProduct<Lhs,Rhs,NestByRefBit>(other.lhs(), other.rhs()); }
 #endif
 
+    ExpressionType& expression() const
+    {
+      return m_expression;
+    }
+
   protected:
     ExpressionType& m_expression;
 };