Do not add non-standard entities to namespace `std`

Reason

Adding to std might change the meaning of otherwise standards conforming code. Additions to std might clash with future versions of the standard.

Example

namespace std { // BAD: violates standard

class My_vector {
    //     . . .
};

}

namespace Foo { // GOOD: user namespace is allowed

class My_vector {
    //     . . .
};

}

Enforcement

Possible, but messy and likely to cause problems with platforms.