Define an order for allMembers traits

[Richard (Rikki) Andrew Cattermole]

Another way to do this (which should be defined):

```d
void main() {
     S s;

     static foreach(f; s.tupleof) {
         pragma(msg, __traits(identifier, f));
     }
}

struct S {
    int a, b;

     union {
         string c, d;
     }
}
```