Combine Xv and XTv and add sparse operator class

Combine Xv and XTv and add sparse operator class

In Hv function, we combine the Xv and XTv operations to
significantly improve the performance.
For detailed analysis, please check Fig. 3 in
http://www.csie.ntu.edu.tw/~cjlin/papers/multicore_liblinear_icdm.pdf
In addition, we replace some repeated linear operations with functions
in the new class, sparse_operator, to improve the readability.