using namespace std; in main headers causes naming conflicts

The `using namespace std;` in the headers is causing naming conflicts.

The reason `std::` exists is to avoid naming conflicts. The fix would be to move all `using namespace std;` declarations into the sources (out of the headers), or not using it at all.

Naming conflicts happen because names from `std::` like `max`, `vector`, `array` can be shadowed by local identifiers with the same names, and some other cases.

Libraries should never do `using namespace std` in headers that may be included by people using the library.

[StackOverflow thread for more information](https://stackoverflow.com/questions/1452721/why-is-using-namespace-std-considered-bad-practice)

No files or tests are named; start by inspecting the library's public headers for `using namespace std;` and the corresponding source files. Done means the declarations are removed from headers, source usage is handled consistently, and the library no longer exposes those namespace-related naming conflicts.

Written by the indexing model from the issue text.