Add feature: comments support

[sweihub]

Hi, added C like comments support, kindly review

#[test]
fn test_comments() {
    assert_eq!(
        eval(
            "
            // input
            a = 1;  // assignment
            // output
            a + 2  // add"
        ),
        Ok(Value::Int(3))
    );

    assert_eq!(
        eval("1 % 4 + /*inline comment*/ 6 /*END*/"),
        Ok(Value::Int(7))
    );

    assert_eq!(
        eval("/* begin */ 10 /* middle */ + 5 /* end */ + 6 // DONE"),
        Ok(Value::Int(21))
    );
}