various work on the Sparse module:

* added some glue to Eigen/Core (SparseBit, ei_eval, Matrix)
* add two new sparse matrix types:
   HashMatrix: based on std::map (for random writes)
   LinkedVectorMatrix: array of linked vectors
   (for outer coherent writes, e.g. to transpose a matrix)
* add a SparseSetter class to easily set/update any kind of matrices, e.g.:
   { SparseSetter<MatrixType,RandomAccessPattern> wrapper(mymatrix);
     for (...) wrapper->coeffRef(rand(),rand()) = rand(); }
* automatic shallow copy for RValue
* and a lot of mess !
plus:
* remove the remaining ArrayBit related stuff
* don't use alloca in product for very large memory allocation

Eigen/src/Core/Matrix.h

@@ -92,7 +92,8 @@ struct ei_traits<Matrix<_Scalar, _Rows, _Cols, _MaxRows, _MaxCols, _Flags> >
                 _Rows, _Cols, _MaxRows, _MaxCols,
                 _Flags
             >::ret,
-    CoeffReadCost = NumTraits<Scalar>::ReadCost
+    CoeffReadCost = NumTraits<Scalar>::ReadCost,
+    SupportedAccessPatterns = RandomAccessPattern
   };
 };