intrusive_ptr aliasing constructor?

Hi all,

I want to ask that is it reliable to implement `intrusive_ptr` aliasing constructor, similar to `shared_ptr` existing one?

Possible usage:
```
struct Test1 : boost::intrusive_ref_counter {
int a;
};

struct Test2 : boost::intrusive_ref_counter {
int b;
};

void fun(boost::intrusive_ptr ctr);

auto iptr1 = boost::intrusive_ptr(new Test1);
auto iptr2 = boost::intrusive_ptr(new Test2);

fun( {iptr1, &iptr1->a} );
fun( {iptr2, &iptr2->b} );
```