v0.2.0

Note that failing test schedules created by versions of Shuttle before 0.2.0 will not successfully
replay on version 0.2.0, and vice versa, as the changes below affect Mutex and RwLock
scheduling decisions.

• Implement Mutex::try_lock (#71)
• Implement RwLock::{try_read, try_write} (#72)
• Export a version of std::sync::Weak (#69)
• Provide better error messages for deadlocks caused by non-reentrant locking (#66)