Ruby

Use a serial to keep track of Mutex-owning Fiber

Previously this held a pointer to the Fiber itself, which requires
marking it (which was only implemented recently, prior to that it was
buggy). Using a monotonically increasing integer instead allows us to
avoid having a free function and keeps everything simpler.

My main motivations in making this change are that the root fiber lazily
allocates self, which makes the writebarrier implementation challenging
to do correctly, and wanting to avoid sending Mutexes to the remembered
set when locked by a short-lived Fiber.