Unsound `Send`/`Sync` of `ArcBorrow`.

[steffahn]

Currently, the implicit

impl<'a, T: ?Sized> Send for ArcBorrow<'a, T>
where
    T: Sync,
impl<'a, T: ?Sized> Sync for ArcBorrow<'a, T>
where
    T: Sync,

don’t match the implementations for &Arc<T> . This is unsound, as T: Send + Sync (not just T: Sync) is essential for payloads dropped in a different thread than the one creating the Arc, as well as for APIs such as Arc::get_mut, etc…

For #65, ArcBorrow needs to be changed anyways, so I suppose this could be fixed at the same time. Of course, that’s then technically a breaking change, but also a soundness fix.