OOM issue in int-to-ascii and int-to-utf8

[Acaccia]

Here are some tests that show the failures of those functions:

#[test]
fn int_to_ascii_oom() {
    crosscheck_oom(
        "(int-to-ascii 42)",
        Ok(Some(
            Value::string_ascii_from_bytes(b"42".to_vec()).unwrap(),
        )),
    );
}

#[test]
fn int_to_utf8_oom() {
    crosscheck_oom(
        "(int-to-utf8 42)",
        Ok(Some(Value::string_utf8_from_bytes(b"42".to_vec()).unwrap())),
    );
}

(These tests will exists after the merging of #586)