When defining a bmethod, we recorded the current Ractor's object in the
method. However that was never marked and so could be GC'd and reused by
a future Ractor. Instead we can use the Ractor's id, which we expect to
be unique forever.

Co-authored-by: Luke Gruber <luke.gru@gmail.com>

Fix race between timer thread dequeuing waiting thread and thread
skipping sleeping due to being dequeued. We now use `th->event_serial` which
is protected by `thread_sched_lock`. When a thread is put on timer thread's waiting
list, the event serial is saved on the item. The timer thread checks
that the saved serial is the same as current thread's serial before
calling `thread_sched_to_ready`.

The following script (taken from a test in `test_thread.rb` used to crash on
scheduler debug assertions. It would likely crash in non-debug mode as well.

```ruby
def assert_nil(val)
  if val != nil
    raise "Expected #{val} to be nil"
  end
end

def assert_equal(expected, actual)
  if expected != actual
    raise "Expected #{expected} to be #{actual}"
  end
end

def test_join2
  ok = false
  t1 = Thread.new { ok = true; sleep }
  Thread.pass until ok
  Thread.pass until t1.stop?
  t2 = Thread.new do
    Thread.pass while ok
    t1.join(0.01)
  end
  t3 = Thread.new do
    ok = false
    t1.join
  end
  assert_nil(t2.value)
  t1.wakeup
  assert_equal(t1, t3.value)
ensure
  t1&.kill&.join
  t2&.kill&.join
  t3&.kill&.join
end

rs = 30.times.map do
  Ractor.new do
    test_join2
  end
end
rs.each(&:join)
```

I organized all examples the followings:

```
Total test time: 2468.41 seconds
Total files: 168

Group A: 42 files, 617.08 seconds
Group B: 42 files, 617.05 seconds
Group C: 42 files, 617.14 seconds
Group D: 42 files, 617.14 seconds
```

ruby/rubygems@94d41e6c7c

… not assigned to any Windows runner group

ruby/rubygems@3ddb740969