C++ headers shouldn't do `using namespace std;`

[sylveon]

Using namespaces in headers is considered bad practice because they unconditionally and irreversibly pollute the global namespace of every consumer.

This leads to conflicts especially since the std namespace uses a lot of general terms that someone might want to use elsewhere, for example function and copy. The user in https://github.com/microsoft/STL/issues/1881 was having issues with a library conflicting with the standard library specifically because of this line (the library is in the endian namespace while C++20 has an std::endian enum now):

https://github.com/justinhj/astar-algorithm-cpp/blob/9b2597e10110d1b8c6a297d52fa06500182d89b5/cpp/stlastar.h#L40

The source .cpp files, however, can keep doing using namespace std since it won't affect any other files.