intrusive_adapter! shoud implement Default and not derive it

[Alexey Galakhov]

Hi,

first, thank you for the great crate. I found a small issue: if I use intrusive_adapter! macro for a type that does not implement Default, it will fail. Actually it does not depend on this Default since it's PhantomData. This is a little bit confusing and requires writing of custom Adapter implementation which does exactly the same as the macro one. I believe an implementation should be used instead of derive. Maybe the same for Clone too, and may be it is even Copy.

[Amanieu d'Antras]

Fixed in 0.7.6