- make MatrixBase and all expressions aware of their preferred traversal order.

Honor this preference in operator=.
- add several methods to the API
- rework API for diagonal matrices
- add benchmarking code

Eigen/src/Core/Block.h

@@ -67,9 +67,10 @@ template<typename MatrixType, int BlockRows, int BlockCols> class Block
     EIGEN_INHERIT_ASSIGNMENT_OPERATORS(Block)
     
   private:
+    static const TraversalOrder _Order = MatrixType::Order;
     static const int _RowsAtCompileTime = BlockRows,
                      _ColsAtCompileTime = BlockCols;
-
+    
     const Block& _ref() const { return *this; }
     int _rows() const { return BlockRows; }
     int _cols() const { return BlockCols; }