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

vm.c

@@ -3557,7 +3557,6 @@ thread_mark(void *ptr)
     rb_gc_mark(th->last_status);
     rb_gc_mark(th->locking_mutex);
     rb_gc_mark(th->name);
-    rb_gc_mark(th->ractor_waiting.receiving_mutex);
 
     rb_gc_mark(th->scheduler);
 
@@ -3719,10 +3718,6 @@ th_init(rb_thread_t *th, VALUE self, rb_vm_t *vm)
     th->ext_config.ractor_safe = true;
 
     ccan_list_head_init(&th->interrupt_exec_tasks);
-    ccan_list_node_init(&th->ractor_waiting.waiting_node);
-#ifndef RUBY_THREAD_PTHREAD_H
-    rb_native_cond_initialize(&th->ractor_waiting.cond);
-#endif
 
 #if USE_RUBY_DEBUG_LOG
     static rb_atomic_t thread_serial = 1;
@@ -4381,7 +4376,8 @@ Init_BareVM(void)
     vm_opt_mid_table = st_init_numtable();
 
 #ifdef RUBY_THREAD_WIN32_H
-    rb_native_cond_initialize(&vm->ractor.sync.barrier_cond);
+    rb_native_cond_initialize(&vm->ractor.sync.barrier_complete_cond);
+    rb_native_cond_initialize(&vm->ractor.sync.barrier_release_cond);
 #endif
 }