Should have @property

[Salih Dincer]

In the code snippet below should have @property. Otherwise, we 
get the result this:

> 40
> NP!(Foo!int)(opps!)
> NP!(Foo!int)(opps!)

```d
struct NP(V) {
   private V payload;
   this(V x)
   {
     payload = x;
   }
   alias opCall this;

   @property: // <-- should be here
   auto opCall() inout // ditto
   {
     return payload.value;
   }
   auto opCall(T)(T x)
   {
     return payload.value = x;
   }
}

class Foo(T)
{
   private T value;
   NP!Foo wrapper;

   this()
   {
     wrapper = NP!Foo(this);
   }

   override string toString()
   {
     return "opps!";
   }
}

import std.stdio;
void main()
{
   auto foo = new Foo!int;
   foo.value = 40;

   foo.value.writeln;   // 40
   foo.wrapper.writeln; // 40

   foo.wrapper = 41;
   foo.wrapper.writeln; // 41
}
```
SDB at 79