Ractor::Port

* Added `Ractor::Port`
  * `Ractor::Port#receive` (support multi-threads)
  * `Rcator::Port#close`
  * `Ractor::Port#closed?`
* Added some methods
  * `Ractor#join`
  * `Ractor#value`
  * `Ractor#monitor`
  * `Ractor#unmonitor`
* Removed some methods
  * `Ractor#take`
  * `Ractor.yield`
* Change the spec
  * `Racotr.select`

You can wait for multiple sequences of messages with `Ractor::Port`.

```ruby
ports = 3.times.map{ Ractor::Port.new }
ports.map.with_index do |port, ri|
  Ractor.new port,ri do |port, ri|
    3.times{|i| port << "r#{ri}-#{i}"}
  end
end

p ports.each{|port| pp 3.times.map{port.receive}}

```

In this example, we use 3 ports, and 3 Ractors send messages to them respectively.
We can receive a series of messages from each port.

You can use `Ractor#value` to get the last value of a Ractor's block:

```ruby
result = Ractor.new do
  heavy_task()
end.value
```

You can wait for the termination of a Ractor with `Ractor#join` like this:

```ruby
Ractor.new do
  some_task()
end.join
```

`#value` and `#join` are similar to `Thread#value` and `Thread#join`.

To implement `#join`, `Ractor#monitor` (and `Ractor#unmonitor`) is introduced.

This commit changes `Ractor.select()` method.
It now only accepts ports or Ractors, and returns when a port receives a message or a Ractor terminates.

We removes `Ractor.yield` and `Ractor#take` because:
* `Ractor::Port` supports most of similar use cases in a simpler manner.
* Removing them significantly simplifies the code.

We also change the internal thread scheduler code (thread_pthread.c):
* During barrier synchronization, we keep the `ractor_sched` lock to avoid deadlocks.
  This lock is released by `rb_ractor_sched_barrier_end()`
  which is called at the end of operations that require the barrier.
* fix potential deadlock issues by checking interrupts just before setting UBF.

https://bugs.ruby-lang.org/issues/21262

thread.c

@@ -526,9 +526,6 @@ thread_cleanup_func(void *th_ptr, int atfork)
     }
 
     rb_native_mutex_destroy(&th->interrupt_lock);
-#ifndef RUBY_THREAD_PTHREAD_H
-    rb_native_cond_destroy(&th->ractor_waiting.cond);
-#endif
 }
 
 static VALUE rb_threadptr_raise(rb_thread_t *, int, VALUE *);
@@ -6174,6 +6171,8 @@ threadptr_interrupt_exec_exec(rb_thread_t *th)
         }
         rb_native_mutex_unlock(&th->interrupt_lock);
 
+        RUBY_DEBUG_LOG("task:%p", task);
+
         if (task) {
             (*task->func)(task->data);
             ruby_xfree(task);
@@ -6228,6 +6227,8 @@ rb_ractor_interrupt_exec(struct rb_ractor_struct *target_r,
 {
     struct interrupt_ractor_new_thread_data *d = ALLOC(struct interrupt_ractor_new_thread_data);
 
+    RUBY_DEBUG_LOG("flags:%d", (int)flags);
+
     d->func = func;
     d->data = data;
     rb_thread_t *main_th = target_r->threads.main;