```ruby
$VERBOSE = true

Ractor.store_if_absent :key do
end #=> warning: the block passed to 'Ractor.store_if_absent' defined at <internal:ractor>:474 may be ignored
```

[Bug #20954]

(forgot to amend...)

to initialize ractor local storage in thread-safety.
[Feature #20875]

Many libraries should be loaded on the main ractor because of
setting constants with unshareable objects and so on.

This patch allows to call `requore` on non-main Ractors by
asking the main ractor to call `require` on it. The calling ractor
waits for the result of `require` from the main ractor.

If the `require` call failed with some reasons, an exception
objects will be deliverred from the main ractor to the calling ractor
if it is copy-able.

Same on `require_relative` and `require` by `autoload`.

Now `Ractor.new{pp obj}` works well (the first call of `pp` requires
`pp` library implicitly).

[Feature #20627]

`Ractor#[]/[]=` is only for accessors to the current ractor, so that
`Ractor.[]/[]=` is simpler.
[Feature #20715]

to return the current ractor is the main ractor.
(== `Ractor.current == Ractor.main`)

`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.