[ruby-core:115999] [Ruby master Bug#20145] Memory leak when duplicating identhash

Issue #20145 has been reported by peterzhu2118 (Peter Zhu).

----------------------------------------
Bug #20145: Memory leak when duplicating identhash
https://bugs.ruby-lang.org/issues/20145

* Author: peterzhu2118 (Peter Zhu)
* Status: Open
* Priority: Normal
* Backport: 3.0: UNKNOWN, 3.1: REQUIRED, 3.2: REQUIRED, 3.3: REQUIRED
----------------------------------------
GitHub PR: https://github.com/ruby/ruby/pull/9411

Both copy_compare_by_id and hash_copy will create a copy of the ST table, so the ST table created in copy_compare_by_id will be leaked.

```ruby
h = { 1 => 2 }.compare_by_identity

10.times do
  1_000_000.times do
    h.select { false }
  end

  puts `ps -o rss= -p #{$$}`
end
```

Before:

```
110736
204352
300272
395520
460704
476736
542000
604704
682624
770528
```

After:

```
15504
16048
16144
16256
16320
16320
16752
16752
16752
16752
```



-- 
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