std::vector: Using emplace_back to replace push_back to improve performance

[nonwill]

As the title.

[aismann]

@AngusJohnson
As I told long time ago great to add it now:
Replace Vector push_back() vs emplace_back() #175

[nonwill]

push_back() vs emplace_back()

The difference between #93f346f and this one is that #93f346f is with less uising of std::move and std::forward which made emplace_back doing almost the same as push_back.