- introduce Part and Extract classes, splitting and extending the former

Triangular class
- full meta-unrolling in Part
- move inverseProduct() to MatrixBase
- compilation fix in ProductWIP: introduce a meta-selector to only do
  direct access on types that support it.
- phase out the old Product, remove the WIP_DIRTY stuff.
- misc renaming and fixes

Eigen/Core

@@ -35,11 +35,7 @@ namespace Eigen {
 #include "src/Core/CwiseBinaryOp.h"
 #include "src/Core/CwiseUnaryOp.h"
 #include "src/Core/CwiseNullaryOp.h"
-#if (defined EIGEN_WIP_PRODUCT)
 #include "src/Core/ProductWIP.h"
-#else
-#include "src/Core/Product.h"
-#endif
 #include "src/Core/Block.h"
 #include "src/Core/Minor.h"
 #include "src/Core/Transpose.h"
@@ -54,7 +50,8 @@ namespace Eigen {
 #include "src/Core/Swap.h"
 #include "src/Core/CommaInitializer.h"
 #include "src/Core/Triangular.h"
-#include "src/Core/TriangularAssign.h"
+#include "src/Core/Part.h"
+#include "src/Core/InverseProduct.h"
 
 } // namespace Eigen