feat: added pub fn sub()

This takes in two u64, and returns the difference.

TESTING WORKFLOWS

example_lib/src/lib.rs

@@ -2,6 +2,10 @@ pub fn add(left: u64, right: u64) -> u64 {
     left + right
 }
 
+pub fn sub(left: u64, right: u64) -> u64 {
+    left - right
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -10,5 +14,7 @@ mod tests {
     fn it_works() {
         let result = add(2, 2);
         assert_eq!(result, 4);
+        let result = sub(2, 2);
+        assert_eq!(result, 0);
     }
 }