How to make a generic function to take a class or struct by reference?

[JN]

I would like to have only one definition of getX if possible, 
because they both are doing the same thing. I can't remove the 
ref one, because without a ref it will pass the struct as a 
temporary and compiler won't like that.

```d
import std.stdio;

struct Foo
{
     int x;

     void doStuff()
     {
         *getX(this) = 5;
     }
}

class Bar
{
     int x;

     void doStuff()
     {
         *getX(this) = 5;
     }
}

int* getX(T)(ref T t)
{
     return &t.x;
}

int* getX(T)(T t)
{
     return &t.x;
}

void main()
{
     Foo foo;
     Bar bar = new Bar();

     foo.doStuff();
     bar.doStuff();

     assert(foo.x == 5);
     assert(bar.x == 5);
}
```