-new: recursive costs system, useful to determine automatically

when to evaluate arguments and when to meta-unroll.
-use it in Product to determine when to eval args. not yet used
 to determine when to unroll. for now, not used anywhere else but
 that'll follow.
-fix badness of my last commit

Eigen/src/Core/DiagonalMatrix.h

@@ -47,7 +47,8 @@ struct ei_traits<DiagonalMatrix<CoeffsVectorType> >
     ColsAtCompileTime = CoeffsVectorType::SizeAtCompileTime,
     MaxRowsAtCompileTime = CoeffsVectorType::MaxSizeAtCompileTime,
     MaxColsAtCompileTime = CoeffsVectorType::MaxSizeAtCompileTime,
-    Flags = CoeffsVectorType::Flags
+    Flags = CoeffsVectorType::Flags,
+    CoeffReadCost = CoeffsVectorType::CoeffReadCost
   };
 };