Use saturating arithmetic on calculating row saving

The saturating arithmetic doesn't overflow, but stays at the maximum or
minimum. In this case it doesn't become negative, but zero, which
satisfies the condition below.

Closes #31

src/main.rs

@@ -229,7 +229,7 @@ fn main() {
     );
 
     if let Some(min) = min_saved_rows {
-        let saving = (original_summed_size - compressed_summed_size) as i32;
+        let saving = original_summed_size.saturating_sub(compressed_summed_size);
         if saving < min {
             println!(
                 "Only {} rows would be saved by this compression. Skipping output.",