feat(modules): update exercises, add modules3 (#822)

Co-authored-by: diannasoriel <mokou@fastmail.com>

exercises/modules/modules3.rs

@@ -0,0 +1,18 @@
+// modules3.rs
+// You can use the 'use' keyword to bring module paths from modules from anywhere
+// and especially from the Rust standard library into your scope.
+// Bring SystemTime and UNIX_EPOCH
+// from the std::time module. Bonus style points if you can do it with one line!
+// Make me compile! Execute `rustlings hint modules3` for hints :)
+
+// I AM NOT DONE
+
+// TODO: Complete this use statement
+use ???
+
+fn main() {
+    match SystemTime::now().duration_since(UNIX_EPOCH) {
+        Ok(n) => println!("1970-01-01 00:00:00 UTC was {} seconds ago!", n.as_secs()),
+        Err(_) => panic!("SystemTime before UNIX EPOCH!"),
+    }
+}