Method create_lock

include/boost/sml.hpp:1874–1881

struct thread_safe : aux::pair<thread_safety_policy__, thread_safe<TLock>> {
  using type = thread_safe;
  constexpr auto create_lock() {
    struct lock_guard {
      TLock &lock_;
      constexpr explicit lock_guard(TLock &lock) : lock_{lock} { lock_.lock(); }
      ~lock_guard() { lock_.unlock(); }
    };
    return lock_guard{lock};
  }
  TLock lock{};
};
}  // namespace policies