SyncRef's clone() and debug() allow data races

Affected versions of this crate unconditionally implement Sync for SyncRef<T>.
This definition allows data races if &T is accessible through &SyncRef.

SyncRef<T> derives Clone and Debug, and the default implementations of those traits access &T by invoking T::clone() & T::fmt(). It is possible to create data races & undefined behavior by concurrently invoking SyncRef<T>::clone() or SyncRef<T>::fmt() from multiple threads with T: !Sync.