fn replace(vm: &mut VM, mut self_val: Value, args: &Args2) -> VMResult {
    args.check_args_num(1)?;
    let self_ = self_val.as_mut_rstring().unwrap();
    let mut arg0 = vm[0];
    *self_ = RString::from(arg0.expect_string("1st arg")?);
    Ok(self_val)
}
String#replace
https://docs.ruby-lang.org/ja/latest/method/String/i/replace.html
ruruby/src/builtin/string.rs