parallelism with delegate

[Vitaliy Fadeev]

able ?
how to use correctly?

```d
   import std.parallelism;

   auto async_task = task!fn( args );  // error
                                       // Error: no property 
`opCall` for type `app.A`, did you mean `new A`?

   async_task.executeInNewThread();
```

where

```d
auto a = new A();
auto fn = &a.download;

class A
{
     void fn( string url )
     {
         // DO
     }
}
```

Playground:
https://run.dlang.io/is/HvhtoP

gist:
https://gist.github.com/run-dlang/218b69e1afd79e5944ea10aa7ca61e1b