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 microsoft/STL#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):

astar-algorithm-cpp/cpp/stlastar.h

Line 40 in 9b2597e

using namespace std;

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

[justinhj]

Thanks for pointing this out, have fixed.