`Ractor::Selector` is not approved by Matz so remove it from
Ruby-level.

The implementation is used by `Ractor.select` so most of implementation
was remaind and calling `rb_init_ractor_selector()`, `Ractor::Selector`
will be defined. I will provide `ractor-selector` gem to try it.

It returns the waiting set is empty or not.

Also add Ractor::Selector's tests.

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.

* Make changes to docs in ractor.rb

Mainly English changes to make things more clear, and to fix minor
non-idiomatic phrases. Also clarified difference between frozen and
shareable objects.

* More minor changes to Ractor docs.

The internal location in ractor.rb is not usefull at all.
```
$ ruby -e 'Ractor.new {}'
<internal:ractor>:267: warning: Ractor is experimental, ...
```

* As the "doc/" prefix is specified by the `--page-dir` option,
  remove from the rdoc references.
* Refer to the original .rdoc instead of the converted .html.