Rewrite Ractor synchronization mechanism

This patch rewrites Ractor synchronization mechanism, send/receive
and take/yield.

* API
  * Ractor::Selector is introduced for lightweight waiting
    for many ractors.
* Data structure
  * remove `struct rb_ractor_waiting_list` and use
    `struct rb_ractor_queue takers_queue` to manage takers.
  * remove `rb_ractor_t::yield_atexit` and use
    `rb_ractor_t::sync::will_basket::type` to check the will.
  * add `rb_ractor_basket::p.take` to represent a taking ractor.
* Synchronization protocol
  * For the Ractor local GC, `take` can not make a copy object
    directly so ask to generate the copy from the yielding ractor.
  * The following steps shows what `r1.take` does on `r0`.
    * step1: (r0) register `r0` into `r1`'s takers.
    * step2: (r0) check `r1`'s status and wakeup r0 if `r1` is waiting
             for yielding a value.
    * step3: (r0) sleep until `r1` wakes up `r0`.
  * The following steps shows what `Ractor.yield(v)` on `r1`.
    * step1: (r1) check first takers of `r1` and if there is (`r0`),
             make a copy object of `v` and pass it to `r0` and
             wakes up `r0`.
    * step2: (r1) if there is no taker ractors, sleep until
             another ractor try to take.

1 files changed, 10 insertions, 4 deletions

diff --git a/bootstraptest/test_ractor.rb b/bootstraptest/test_ractor.rb
index 459ba260e9..6898edc677 100644
--- a/bootstraptest/test_ractor.rb
+++ b/bootstraptest/test_ractor.rb
@@ -518,9 +518,9 @@ assert_equal '[true, true, true]', %q{
     end
   }
   received = []
-  take = []
+  taken = []
   yielded = []
-  until rs.empty?
+  until received.size == RN && taken.size == RN && yielded.size == RN
     r, v = Ractor.select(CR, *rs, yield_value: 'yield')
     case r
     when :receive
@@ -528,11 +528,17 @@ assert_equal '[true, true, true]', %q{
     when :yield
       yielded << v
     else
-      take << v
+      taken << v
       rs.delete r
     end
   end
-  [received.all?('sendyield'), yielded.all?(nil), take.all?('take')]
+  r = [received == ['sendyield'] * RN,
+       yielded  == [nil] * RN,
+       taken    == ['take'] * RN,
+  ]
+
+  STDERR.puts [received, yielded, taken].inspect
+  r
 }
 
 # multiple Ractors can send to one Ractor