To lower case

#[test]
fn t1() {
    let s = std::time::Instant::now();
    let v = Solution::to_lower_case("Hello".to_string());
    log_result!(v == "hello", "t1", s, "hello", v);
    assert_eq!(v, "hello");
}

#[test]
fn t2() {
    let s = std::time::Instant::now();
    let v = Solution::to_lower_case("here".to_string());
    log_result!(v == "here", "t2", s, "here", v);
    assert_eq!(v, "here");
}

#[test]
fn t3() {
    let s = std::time::Instant::now();
    let v = Solution::to_lower_case("LOVELY".to_string());
    log_result!(v == "lovely", "t3", s, "lovely", v);
    assert_eq!(v, "lovely");
}

#[test]
fn t4() {
    let s = std::time::Instant::now();
    let v = Solution::to_lower_case("A".to_string());
    log_result!(v == "a", "t4", s, "a", v);
    assert_eq!(v, "a");
}

#[test]
fn t5() {
    let s = std::time::Instant::now();
    let v = Solution::to_lower_case("aAbBcC".to_string());
    log_result!(v == "aabbcc", "t5", s, "aabbcc", v);
    assert_eq!(v, "aabbcc");
}