Suggest get_mut() instead of lock() method on Mutex

[kryptan]

If you have mut reference to a Mutex then get_mut method is more efficient than lock since it doesn't require locking. If clippy detects such situation it could suggest to use get_mut instead of lock.

[oli-obk]

You need to use expr_ty instead of expr_ty_adjusted on the self argument of the lock call and check whether it's a mutable reference or a name of something that's mutable.

We can't detect whether we have immutable ownership to the value. so let foo = Mutex::new(...); foo.lock().... won't trigger :(