Registers filename to be loaded (using Kernel::require) the first time that
module (which may be a String or a
symbol) is accessed in the namespace of mod.
static VALUE
rb_mod_autoload(VALUE mod, VALUE sym, VALUE file)
{
ID id = rb_to_id(sym);
FilePathValue(file);
rb_autoload_str(mod, id, file);
return Qnil;
}