* Draft of a eigenvalues solver

(does not support complex and does not re-use the QR decomposition)

* Rewrite the cache friendly product to have only one instance per scalar type !
  This significantly speeds up compilation time and reduces executable size.
  The current drawback is that some trivial expressions might be
  evaluated like conjugate or negate.

* Renamed "cache optimal" to "cache friendly"

* Added the ability to directly access matrix data of some expressions via:
  - the stride()/_stride() methods
  - DirectAccessBit flag (replace ReferencableBit)

Eigen/src/Core/Transpose.h

@@ -69,6 +69,8 @@ template<typename MatrixType> class Transpose
     int _rows() const { return m_matrix.cols(); }
     int _cols() const { return m_matrix.rows(); }
 
+    int _stride(void) const { return m_matrix.stride(); }
+
     Scalar& _coeffRef(int row, int col)
     {
       return m_matrix.const_cast_derived().coeffRef(col, row);