C++ named requirements: Lockable (since C++11)

The Lockable requirements extends the BasicLockable requirements to include attempted locking.

For type L to be Lockable, it must meet the above condition as well as the following:

Expression	Effects	Return value
`m.try_lock()`	Attempts to acquire the lock for the current execution agent (thread, process, task) without blocking. If an exception is thrown, no lock is obtained.	`true` if the lock was acquired, `false` otherwise

The try_lock member functions obtains a non-shared lock on m on success.

The following standard library types satisfy Lockable requirements:

mutex (C++11)	provides basic mutual exclusion facility (class)
recursive_mutex (C++11)	provides mutual exclusion facility which can be locked recursively by the same thread (class)
recursive_timed_mutex (C++11)	provides mutual exclusion facility which can be locked recursively by the same thread and implements locking with a timeout (class)
shared_mutex (C++17)	provides shared mutual exclusion facility (class)
shared_timed_mutex (C++14)	provides shared mutual exclusion facility and implements locking with a timeout (class)
timed_mutex (C++11)	provides mutual exclusion facility which implements locking with a timeout (class)