Add rb_ident_set_new internal function

This is designed to replace rb_ident_hash_new calls when such hashes
are used purely with with true values.

To implement this, add a set_alloc_with_size_and_type static function,
and have set_alloc_with_size and rb_ident_set_new call it.

set.c

@@ -395,17 +395,22 @@ set_insert_wb(VALUE set, VALUE key)
 }
 
 static VALUE
-set_alloc_with_size(VALUE klass, st_index_t size)
+set_alloc_with_size_and_type(VALUE klass, st_index_t size, const struct st_hash_type *type)
 {
     VALUE set;
     struct set_object *sobj;
 
     set = TypedData_Make_Struct(klass, struct set_object, &set_data_type, sobj);
-    set_init_table_with_size(&sobj->table, &objhash, size);
+    set_init_table_with_size(&sobj->table, type, size);
 
     return set;
 }
 
+static VALUE
+set_alloc_with_size(VALUE klass, st_index_t size)
+{
+    return set_alloc_with_size_and_type(klass, size, &objhash);
+}
 
 static VALUE
 set_s_alloc(VALUE klass)
@@ -2264,6 +2269,14 @@ compat_loader(VALUE self, VALUE a)
     return set_i_from_hash(self, rb_ivar_get(a, id_i_hash));
 }
 
+/* Internal C-API functions */
+
+VALUE
+rb_ident_set_new(void)
+{
+    return set_alloc_with_size_and_type(rb_cSet, 0, &identhash);
+}
+
 /* C-API functions */
 
 void