GenericMutexGuard allows data races of non-Sync types across threads

GenericMutexGuard<T> was given the Sync auto trait as long as T is Send
due to its contained members. However, since the guard is supposed to represent
an acquired lock and allows concurrent access to the underlying data from
different threads, it should only be Sync when the underlying data is.

This is a soundness issue and allows data races, potentially leading to crashes
and segfaults from safe Rust code.

The flaw was corrected by adding a T: Send + Sync bound for
GenericMutexGuard’s Sync trait.

This bug is similar to one in
std::sync::Mutex.