Add rb_class_get_superclass function

Author: andrykonchin

lib/cext/ABI_check.txt

@@ -1 +1 @@
-16
+17

lib/truffle/truffle/cext.rb

@@ -603,6 +603,10 @@ def rb_class_real(ruby_class)
     ruby_class
   end
 
+  def rb_class_get_superclass(ruby_class)
+    ruby_class.superclass || false
+  end
+
   def rb_obj_respond_to(object, name, priv)
     object.respond_to?(name, priv != 0)
   end

src/main/c/cext/class.c

@@ -51,6 +51,10 @@ VALUE rb_singleton_class(VALUE object) {
   return RUBY_CEXT_INVOKE("rb_singleton_class", object);
 }
 
+VALUE rb_class_get_superclass(VALUE klass) {
+  return RUBY_CEXT_INVOKE("rb_class_get_superclass", klass);
+}
+
 VALUE rb_obj_alloc(VALUE ruby_class) {
   return RUBY_INVOKE(ruby_class, "__allocate__");
 }