* find the proper way of nesting the expression in Flagged:

finally that's more subtle than just using ei_nested, because when
  flagging with NestByValueBit we want to store the expression by value
  already, regardless of whether it already had the NestByValueBit set.
* rename temporary() ----> nestByValue()
* move the old Product.h to disabled/, replace by what was ProductWIP.h
* tweak -O and -g flags for tests and examples
* reorder the tests -- basic things go first
* simplifications, e.g. in many methoeds return derived() and count on
  implicit casting to the actual return type.
* strip some not-really-useful stuff from the heaviest tests

Eigen/src/Core/Flagged.h

@@ -33,7 +33,7 @@
   * \param Added the flags added to the expression
   * \param Removed the flags removed from the expression (has priority over Added).
   *
-  * This class represents an expression whose flags have been modified
+  * This class represents an expression whose flags have been modified.
   * It is the return type of MatrixBase::flagged()
   * and most of the time this is the only way it is used.
   *
@@ -94,7 +94,11 @@ template<typename ExpressionType, unsigned int Added, unsigned int Removed> clas
     }
 
   protected:
-    const ExpressionType m_matrix;
+    const typename ei_meta_if<
+      Added & ~Removed & NestByValueBit,
+      ExpressionType,
+      typename ExpressionType::Nested
+    >::ret m_matrix;
 };
 
 /** \returns an expression of *this with added flags
@@ -121,7 +125,7 @@ MatrixBase<Derived>::lazy() const
   */
 template<typename Derived>
 inline const Flagged<Derived, NestByValueBit, 0>
-MatrixBase<Derived>::temporary() const
+MatrixBase<Derived>::nestByValue() const
 {
   return derived();
 }