Rework error_code internal data members

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.