[ruby-core:116078] [Ruby master Bug#20162] Memory leak when duplicating too complex object

Issue #20162 has been reported by peterzhu2118 (Peter Zhu).

----------------------------------------
Bug #20162: Memory leak when duplicating too complex object
https://bugs.ruby-lang.org/issues/20162

* Author: peterzhu2118 (Peter Zhu)
* Status: Open
* Priority: Normal
* Backport: 3.0: UNKNOWN, 3.1: DONTNEED, 3.2: REQUIRED, 3.3: REQUIRED
----------------------------------------
GitHub PR: https://github.com/ruby/ruby/pull/9449

Creating a ST table then calling st_replace leaks memory because the st_replace overwrites the ST table without freeing any of the existing memory. This commit changes it to use st_copy instead.

For example:

```ruby
RubyVM::Shape.exhaust_shapes

o = Object.new
o.instance_variable_set(:@a, 0)

10.times do
  100_000.times { o.dup }

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

Before:

```
23264
33600
42672
52160
61600
71728
81056
90528
100560
109840
```

After:

```
14752
14816
15584
15584
15664
15664
15664
15664
15664
15664
```



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