Remove "using namespace std"

[yasushi-saito]

It breaks builds using clang + libc++ + c++17.

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[CarloWood]

Haha, I made completely independent the exact same commit! See,
CarloWood@61c7939
Only found this after I came here to add a pull request ;).

[cdluminate]

Thanks for this patch!

[CarloWood]

Sad world that this is STILL not merged. I can no longer use this repository because of this.
I guess I'll be forced to keep a clone and maintain that :/