Add {Sync, Send} on ArcCell

[icorderi]

Shouldn't ArcCell have impl {Sync, Send}?

I guess T has to be Send:

unsafe impl<T: Send> Sync for ArcCell<T> {}
unsafe impl<T: Send> Send for ArcCell<T> {}

[ghost]

This is the definition of the struct:

pub struct ArcCell<T>(AtomicUsize, PhantomData<Arc<T>>);

ArcCell simply inherits the constraints of Arc<T> -- it is Sync + Send if T: Send + Sync.

[Vtec234]

While it is true that the inherited traits are correct, it might be worth it to consider explicitly impling or !impl ing Send and Sync for all types in crossbeam, given the finnicky nature of concurrent programming. An example of when implicitness backfired is rust-lang/rust#41622.

[ghost]

@Vtec234 I agree. Also, it's good to have explicit impls of Send and Sync in the generated docs.