add overflow example

example/standalone/overflow_symbolic.rs

+fn main() {
+    use std::io::{stdin, Read};
+    let mut data = [0; 1];
+    stdin().read_exact(&mut data[..]).unwrap();
+    let i = data[0] as i8;
+    let _overflowed: i8 = i - 1i8;
+}