[ruby-core:101894] [Ruby master Bug#17506] Ractor isolation broken by ThreadGroup

Issue #17506 has been reported by marcandre (Marc-Andre Lafortune).

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Bug #17506: Ractor isolation broken by ThreadGroup
https://bugs.ruby-lang.org/issues/17506

* Author: marcandre (Marc-Andre Lafortune)
* Status: Open
* Priority: Normal
* ruby -v: ruby 3.0.0p0 (2020-12-25 revision 95aff21468) [x86_64-darwin18]
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN
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Ractors currently share the ThreadGroup.

This doesn't seem very useful as there is no possible communication between the Threads of different Ractors.

It is also an isolation error:
```ruby
ThreadGroup.attr_accessor :foo
var = Thread.current.group.foo = [:example]
Ractor.new { Thread.current.group.foo << [:oops] }.take
var # => [:example, [:oops]]
```

Should `Ractor.new` create a new `ThreadGroup`? Should `ThreadGroup` not have (non-shareable) instance variables? Or should `Ractor.new { Thread.current.group }.take` be `nil`? See also https://bugs.ruby-lang.org/issues/17505 about `nil`.


Note that `Ractor` respects the `ThreadGroup`'s state:

```ruby
Thread.current.group.enclose
Ractor.new { Thread.current.group.add(Thread.current) }.take # => can't move to the enclosed thread group

Thread.current.group.freeze
Ractor.new {} # => ThreadError (can't start a new thread (frozen ThreadGroup))
```

I am not sure what is the best behavior as I don't have enough experience with how ThreadGroups are used, especially enclosed ThreadGroups.



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