* add bench/benchVecAdd.cpp by Gael, fix crash (ei_pload on non-aligned)

* introduce packet(int), make use of it in linear vectorized paths
  --> completely fixes the slowdown noticed in benchVecAdd.
* generalize coeff(int) to linear-access xprs
* clarify the access flag bits
* rework api dox in Coeffs.h and util/Constants.h
* improve certain expressions's flags, allowing more vectorization
* fix bug in Block: start(int) and end(int) returned dyn*dyn size
* fix bug in Block: just because the Eval type has packet access
  doesn't imply the block xpr should have it too.

Eigen/src/Core/Map.h

@@ -80,6 +80,16 @@ template<typename MatrixType> class Map
         return const_cast<Scalar*>(m_data)[row + col * m_rows];
     }
 
+    inline const Scalar& _coeff(int index) const
+    {
+      return m_data[index];
+    }
+
+    inline Scalar& _coeffRef(int index)
+    {
+      return m_data[index];
+    }
+
   public:
     inline Map(const Scalar* data, int rows, int cols) : m_data(data), m_rows(rows), m_cols(cols)
     {