* replaced some unordered_map::insert(std::make_pair(foo, bar)) with unordered_map::emplace(foo, bar)
* replaced some vector::push_back(std::make_pair(foo, bar)) with vector::emplace_back(foo, bar)
The old way will construct a std::pair first then call move constructor
when putting it into the container, while using emplace will construct
the pair in-place in the container. Also, the emplace way is shorter &
more concise.