Fix memory leak when copying ST tables

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
```

1 files changed, 2 insertions, 0 deletions

diff --git a/include/ruby/st.h b/include/ruby/st.h
index 0d42283364..ba69c066c9 100644
--- a/include/ruby/st.h
+++ b/include/ruby/st.h
@@ -162,6 +162,8 @@ void rb_st_cleanup_safe(st_table *, st_data_t);
 #define st_cleanup_safe rb_st_cleanup_safe
 void rb_st_clear(st_table *);
 #define st_clear rb_st_clear
+st_table *rb_st_replace(st_table *new_tab, st_table *old_tab);
+#define st_replace rb_st_replace
 st_table *rb_st_copy(st_table *);
 #define st_copy rb_st_copy
 CONSTFUNC(int rb_st_numcmp(st_data_t, st_data_t));