Arc::from_inner / Rc::from_inner should probably be marked unsafe

[sdroege]

See

rust/library/alloc/src/sync.rs

Lines 255 to 257 in 0c87288

	fn from_inner(ptr: NonNull<ArcInner<T>>) -> Self {
	Self { ptr, phantom: PhantomData }
	}

This can be called like

let dangling_arc = Arc::from_inner(NonNull::dangling());

without any unsafe code and results in an Arc to a dangling pointer, which will cause fun when trying to use it.