alias this and opDot()

[Salih Dincer]

Ok, opDot() is deprecated in version 2.082. However, if we 
simulate this situation with a function (opdot in our example), 
we get unexpected results.

```d
struct Point { int x, y; }

struct S
{
     Point point;
     alias opdot this;

     auto opdot()
     { // opDot() is deprecated in v2.082
       //this.point = Point(100, 100);/* <-- also if you only use: 
s == Point(100, 101)
         point.x = 100;
         point.y = 100;
         point.x++;//*/

         return &point;
     }
}

void main()
{
     S s;
     s.x++; // as if this line doesn't exist!
     s.y++;

     //s.x++; //  <-- also if you use: s == Point(102, 100)
     assert(s.point == Point(101, 101));
}
```

SDB at 79