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, 1 insertions, 0 deletions

diff --git a/parser_st.h b/parser_st.h
index 85db3b5847..877b1e9051 100644
--- a/parser_st.h
+++ b/parser_st.h
@@ -137,6 +137,7 @@ void rb_parser_st_add_direct(parser_st_table *, parser_st_data_t, parser_st_data
 void rb_parser_st_free_table(parser_st_table *);
 void rb_parser_st_cleanup_safe(parser_st_table *, parser_st_data_t);
 void rb_parser_st_clear(parser_st_table *);
+parser_st_table *rb_parser_st_replace(parser_st_table *, parser_st_table *);
 parser_st_table *rb_parser_st_copy(parser_st_table *);
 CONSTFUNC(int rb_parser_st_numcmp(parser_st_data_t, parser_st_data_t));
 CONSTFUNC(parser_st_index_t rb_parser_st_numhash(parser_st_data_t));