Introduce free function to rb_concurrent_set_funcs

If we create a key but don't insert it (due to other Ractor winning the
race), then it would leak memory if we don't free it. This introduces a
new function to free that memory for this case.

concurrent_set.c

@@ -304,6 +304,14 @@ rb_concurrent_set_find_or_insert(VALUE *set_obj_ptr, VALUE key, void *data)
             if (set->funcs->cmp(key, curr_key)) {
                 // We've found a match.
                 RB_GC_GUARD(set_obj);
+
+                if (inserting) {
+                    // We created key using set->funcs->create, but we didn't end
+                    // up inserting it into the set. Free it here to prevent memory
+                    // leaks.
+                    if (set->funcs->free) set->funcs->free(key);
+                }
+
                 return curr_key;
             }
 

internal/concurrent_set.h

@@ -4,14 +4,11 @@
 #include "ruby/atomic.h"
 #include "ruby/ruby.h"
 
-typedef VALUE (*rb_concurrent_set_hash_func)(VALUE key);
-typedef bool (*rb_concurrent_set_cmp_func)(VALUE a, VALUE b);
-typedef VALUE (*rb_concurrent_set_create_func)(VALUE key, void *data);
-
 struct rb_concurrent_set_funcs {
-    rb_concurrent_set_hash_func hash;
+    VALUE (*hash)(VALUE key);
-    rb_concurrent_set_cmp_func cmp;
+    bool (*cmp)(VALUE a, VALUE b);
-    rb_concurrent_set_create_func create;
+    VALUE (*create)(VALUE key, void *data);
+    void (*free)(VALUE key);
 };
 
 VALUE rb_concurrent_set_new(const struct rb_concurrent_set_funcs *funcs, int capacity);

string.c

@@ -552,6 +552,7 @@ static const struct rb_concurrent_set_funcs fstring_concurrent_set_funcs = {
     .hash = fstring_concurrent_set_hash,
     .cmp = fstring_concurrent_set_cmp,
     .create = fstring_concurrent_set_create,
+    .free = NULL,
 };
 
 void

symbol.c

@@ -315,10 +315,19 @@ sym_set_create(VALUE sym, void *data)
     }
 }
 
+static void
+sym_set_free(VALUE sym)
+{
+    if (sym_set_sym_static_p(sym)) {
+        xfree(sym_set_static_sym_untag(sym));
+    }
+}
+
 static const struct rb_concurrent_set_funcs sym_set_funcs = {
     .hash = sym_set_hash,
     .cmp = sym_set_cmp,
     .create = sym_set_create,
+    .free = sym_set_free,
 };
 
 static VALUE