- 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/DiagonalCoeffs.h

@@ -51,6 +51,7 @@ template<typename MatrixType> class DiagonalCoeffs
     EIGEN_INHERIT_ASSIGNMENT_OPERATORS(DiagonalCoeffs)
     
   private:
+    static const TraversalOrder _Order = ColumnMajor;
     static const int _RowsAtCompileTime = MatrixType::RowsAtCompileTime,
                      _ColsAtCompileTime = 1;