zip_iterator specialization for std::pair

Signed-off-by: Kohei Takahashi <flast@flast.jp>
make_zip_iterator should be inlined

Signed-off-by: Kohei Takahashi <flast@flast.jp>
Remove unnecessary specialization

Signed-off-by: Kohei Takahashi <flast@flast.jp>
Add tests for fusion based zip_iterator

Signed-off-by: Kohei Takahashi <flast@flast.jp>