Downstream crates may implement `Copy` for `Box<_>`

[dhm]

It looks like the soundness hole of Pin started moving things in the direction of stdlib's explicit negative impls:

@nikomatsakis

I am not sure how far we will want to go, but I think at least the first three bullets. In particular, I would like for us to be able to do things like:

impl<T> !Copy for Box<T> { }

which in turn would mean you could do things like:

trait MyTrait { }
impl<T: Copy> MyTrait for T { }
impl<T> MyTrait for Box<T> { }

---

Tangentially, I recently stumbled upon a related error which is also quite annoying:

trait PrivateTrait {}

pub(whatever)
trait MyTrait { ... }

impl<T : PrivateTrait> MyTrait for T { ... }
impl<Referee : ?Sized> MyTrait for &'_ Referee { ... }

failing with downstream crates may implement PrivateTrait for &'_ _!