- 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/Transpose.h

@@ -51,6 +51,8 @@ template<typename MatrixType> class Transpose
     EIGEN_INHERIT_ASSIGNMENT_OPERATORS(Transpose)
     
   private:
+    static const TraversalOrder _Order = (MatrixType::Order == ColumnMajor)
+                                       ? RowMajor : ColumnMajor;
     static const int _RowsAtCompileTime = MatrixType::ColsAtCompileTime,
                      _ColsAtCompileTime = MatrixType::RowsAtCompileTime;