Better Debug for Mutex

[MikailBag]

Motivation

Before it looked like

Mutex {
    s: Semaphore,
    d: UnsafeCell
}

which is quite useless.
New output is

Mutex {
    inner: X
}

Where X is determined by locking the mutex. If we succeed, we call Debug for inner valuer. Otherwise, X := "<locked>".

IMO proposed Debug output is strictly better than current.

Alternative: always lock mutex and show inner value. However, such behavior can be rather unexpected so I implemented something more conservative.

[MikailBag]

Same should be done for RwLock, but it misses try_read method currently.

[hawkw]

I'm definitely in favor of making the formatted output more user-friendly.

I had a couple notes on changes we might want to make for consistency with how std::sync::Mutex implements Debug. What do you think?

[MikailBag]

@hawkw thanks for your review! I've made both improvements and added a test.

[hawkw]

Looks good to me!