Add a *very efficient* evaluation path for both col-major matrix * vector

and vector * row-major products. Currently, it is enabled only is the matrix
has DirectAccessBit flag and the product is "large enough".
Added the respective unit tests in test/product/cpp.

Eigen/Core

@@ -59,10 +59,7 @@ namespace Eigen {
 #include "src/Core/CommaInitializer.h"
 #include "src/Core/Extract.h"
 #include "src/Core/Part.h"
-
-#ifndef EIGEN_EXTERN_INSTANTIATIONS
 #include "src/Core/CacheFriendlyProduct.h"
-#endif
 
 } // namespace Eigen