Rework error_code internal data members

[pdimov]

Instead of

    struct data
    {
        int val_;
        const error_category * cat_;
    };

    union
    {
        data d1_;
        unsigned char d2_[ sizeof(std::error_code) ];
    };

it might be better to have

int val_;
void const* cat_; // error_category or std::error_category

which could both simplify the code (esp. when we transition the std::error_code case to (value, unknown_category)) and help with GCC false -Wmaybe-uninitialized positives.