[ruby-core:105573] [Ruby master Bug#18243] Ractor.make_shareable does not freeze the receiver of a Proc but allows accessing ivars of it

Issue #18243 has been reported by Eregon (Benoit Daloze).

----------------------------------------
Bug #18243: Ractor.make_shareable does not freeze the receiver of a Proc but allows accessing ivars of it
https://bugs.ruby-lang.org/issues/18243

* Author: Eregon (Benoit Daloze)
* Status: Open
* Priority: Normal
* Backport: 2.6: UNKNOWN, 2.7: UNKNOWN, 3.0: UNKNOWN
----------------------------------------
```ruby
c = C.new
c.foo = 1
p c
proc = c.setter_proc
p c.frozen?
Ractor.new(proc) { |s| s.call(42) }.take
p c
```
gives
```
#<C:0x00007f2a3aae7a98 @foo=1>
false
#<C:0x00007f2a3aae7a98 @foo=42> # BUG
```

But that must be a bug, it means the non-main Ractor can directly mutate an object from the main Ractor.

I found this while thinking about https://github.com/ruby/ostruct/pull/29/files and
whether `Ractor.make_shareable` would freeze `@table` and the `OpenStruct` instance (I think it needs to).

Repro code for ostruct and changing ostruct.rb to `$setter = ::Ractor.make_shareable(setter_proc)`:
```ruby
require 'ostruct'

os = OpenStruct.new
os.foo = 1

$setter.call(2)
p os

Ractor.new($setter) { |s| s.call(42) }.take
p os
```
gives
```
#<OpenStruct foo=2>
<internal:ractor>:267: warning: Ractor is experimental, and the behavior may change in future versions of Ruby! Also there are many implementation issues.
#<OpenStruct foo=42> # BUG
```



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