feature(fiber): add fiber::Mutex - analogue to std::sync::Mutex

the api mirrors that of std::sync::Mutex with a couple of excpetions:

• no way to check if the mutex is poisoned, because a panic inside of a fiber is impossible to catch for the moment
• Mutex::try_lock will abort (in a tarantool debug build) if called from a fiber who already holds the lock. Fixing this would require to delve into box_latch_t internals, which can be changed in future versions of tarantool