bug or feature? shared objects and tuples

[Jack Applegame]

It is impossible to pack a structure with shared object into 
tuple.

```
import std.concurrency;
import std.typecons;

class Foo {}

struct A {
	shared Foo foo;
}

void main() {
	auto a = tuple(new shared Foo); // ОК
  	auto b = tuple(A());            // Error: static assert "unable 
to format shared objects"
}
```