Add a rwlock method to report if lock is held exclusively

[declanvk]

Closes issue #293.

I also ran into the same issue posed in issue #293. Specifically, it was
for implementing an optimistic hybrid lock as described in this paper
in section 2.3, listing #2.

[Amanieu]

I think you could add a default implementation in the trait which attempts to get a shared lock. This should only fail if there is an active exclusive lock. The default implementation is needed to maintain backwards compatibility with existing implementations of this trait.

[declanvk]

I think you could add a default implementation in the trait which attempts to get a shared lock. This should only fail if there is an active exclusive lock. The default implementation is needed to maintain backwards compatibility with existing implementations of this trait.

I'll add that to the trait definition. To test for an exclusive lock, would it look something like:

    fn is_locked_exclusive(&self) -> bool {
        let acquired_lock = self.try_lock_shared();
        if acquired_lock {
            // Safety: A shared lock was successfully acquired above.
            unsafe {
                self.unlock_shared();
            }
        }
        !acquired_lock
    }

to parallel the existing is_locked default implementation?

[Amanieu]

That's correct.