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
```

parser_st.c

@@ -109,6 +109,8 @@ nonempty_memcpy(void *dest, const void *src, size_t n)
 #define st_add_direct rb_parser_st_add_direct
 #undef st_insert2
 #define st_insert2 rb_parser_st_insert2
+#undef st_replace
+#define st_replace rb_parser_st_replace
 #undef st_copy
 #define st_copy rb_parser_st_copy
 #undef st_delete_safe