Implement Write Barrier for `autoload_data`

It's not uncommon for libraries to add thing sinto
autoload that won't necessarily be loaded.

This can cause hundreds or thousands of entries to be
left over in the autoload table, so it's best not to
mark them on every minor.

variable.c

@@ -2212,7 +2212,7 @@ autoload_data_memsize(const void *ptr)
 static const rb_data_type_t autoload_data_type = {
     "autoload_data",
     {autoload_data_mark, autoload_data_free, autoload_data_memsize, autoload_data_compact},
-    0, 0, RUBY_TYPED_FREE_IMMEDIATELY
+    0, 0, RUBY_TYPED_FREE_IMMEDIATELY | RUBY_TYPED_WB_PROTECTED
 };
 
 static void
@@ -2263,11 +2263,12 @@ get_autoload_data(VALUE autoload_const_value, struct autoload_const **autoload_c
 {
     struct autoload_const *autoload_const = rb_check_typeddata(autoload_const_value, &autoload_const_type);
 
-    struct autoload_data *autoload_data = rb_check_typeddata(autoload_const->autoload_data_value, &autoload_data_type);
+    VALUE autoload_data_value = autoload_const->autoload_data_value;
+    struct autoload_data *autoload_data = rb_check_typeddata(autoload_data_value, &autoload_data_type);
 
     /* do not reach across stack for ->state after forking: */
     if (autoload_data && autoload_data->fork_gen != GET_VM()->fork_gen) {
-        autoload_data->mutex = Qnil;
+        RB_OBJ_WRITE(autoload_data_value, &autoload_data->mutex, Qnil);
         autoload_data->fork_gen = 0;
     }
 
@@ -2306,8 +2307,8 @@ autoload_feature_lookup_or_create(VALUE feature, struct autoload_data **autoload
 
     if (NIL_P(autoload_data_value)) {
         autoload_data_value = TypedData_Make_Struct(0, struct autoload_data, &autoload_data_type, autoload_data);
-        autoload_data->feature = feature;
-        autoload_data->mutex = Qnil;
+        RB_OBJ_WRITE(autoload_data_value, &autoload_data->feature, feature);
+        RB_OBJ_WRITE(autoload_data_value, &autoload_data->mutex, Qnil);
         ccan_list_head_init(&autoload_data->constants);
 
         if (autoload_data_pointer) *autoload_data_pointer = autoload_data;
@@ -2608,7 +2609,7 @@ autoload_load_needed(VALUE _arguments)
     }
 
     if (NIL_P(autoload_data->mutex)) {
-        autoload_data->mutex = rb_mutex_new();
+        RB_OBJ_WRITE(autoload_const->autoload_data_value, &autoload_data->mutex, rb_mutex_new());
         autoload_data->fork_gen = GET_VM()->fork_gen;
     }
     else if (rb_mutex_owned_p(autoload_data->mutex)) {