-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/Block.h

@@ -69,7 +69,8 @@ struct ei_traits<Block<MatrixType, BlockRows, BlockCols> >
       : (BlockCols==Dynamic ? MatrixType::MaxColsAtCompileTime : BlockCols),
     Flags = RowsAtCompileTime == Dynamic || ColsAtCompileTime == Dynamic
             ? (unsigned int)MatrixType::Flags
-            : (unsigned int)MatrixType::Flags &~ LargeBit
+            : (unsigned int)MatrixType::Flags &~ LargeBit,
+    CoeffReadCost = MatrixType::CoeffReadCost
   };
 };