hehe, the complicated nesting scheme in Flagged in the previous commit

was a sign that we were doing something wrong. In fact, having
NestByValue as a special case of Flagged was wrong, and the previous
commit, while not buggy, was inefficient because then when the resulting
NestByValue xpr was nested -- hence copied -- the original xpr which was
already nested by value was copied again; hence instead of 1 copy we got
3 copies.
The solution was to ressuscitate the old Temporary.h (renamed
NestByValue.h) as it was the right approach.

Eigen/src/Core/util/ForwardDeclarations.h

@@ -35,6 +35,7 @@ template<typename _Scalar, int _Rows, int _Cols,
 class Matrix;
 
 template<typename ExpressionType, unsigned int Added, unsigned int Removed> class Flagged;
+template<typename ExpressionType> class NestByValue;
 template<typename MatrixType> class Minor;
 template<typename MatrixType, int BlockRows=Dynamic, int BlockCols=Dynamic> class Block;
 template<typename MatrixType> class Transpose;