Operator Overloading - Only for Classes and Structs?

[Andy Valencia]

I was a little surprised the subclassing syntax didn't do 
structural concatenation for struct's.  That is,

```d
import std.stdio : writeln;

struct A {
  int a;
  void printA() {
   println(this.a);
  }
}
struct B : A {
  int b;
  void printAB() {
   this.printA();
   println(this.b);
  }
}

void an_A_fun(A arg) {
  arg.printA();
}

void main() {
  B foo;
  foo.a = 1;
  foo.b = 2;
  foo.printAB();
  an_A_fun(foo);
}
```

Unlike classes, this would guarantee storage adjacency, and still 
preserve its behavior as a value rather than an object reference. 
  And then you get to share code for common initial parts of a 
given struct.