Test-And-Test-And-Set lock

## ttas_mutex.cpp
#include <thread>
#include <atomic>

class ttas_mutex {
  std::atomic<bool> locked_;
public:
  ttas_mutex()
    : locked_(false) {}
  ttas_mutex(const ttas_mutex&) = delete;
  ttas_mutex& operator=(const ttas_mutex&) = delete;

  void lock() {
    for (;;) {
      bool state = false;
      if (locked_.compare_exchange_weak(state, true, memory_order_acquire))
        break;
      while (locked_.load(memory_order_relaxed) == state)
        std::this_thread::yield();
    }
  }
  void unlock() {
    locked_.store(false, memory_order_release);
  }
};