st_copy allocates a st_table, which is not needed for hashes since it is
allocated by VWA and embedded, so this causes a memory leak.

The following script demonstrates the issue:

```ruby
20.times do
  100_000.times do
    {a: 1, b: 2, c: 3, d: 4, e: 5, f: 6, g: 7, h: 8, i: 9}
  end

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