feat: add lifetimes exercises

exercises/lifetimes/lifetimes2.rs

@@ -0,0 +1,27 @@
+// lifetimes2.rs
+//
+// So if the compiler is just validating the references passed
+// to the annotated parameters and the return type, what do
+// we need to change?
+//
+// Execute `rustlings hint lifetimes2` or use the `hint` watch subcommand for a hint.
+
+// I AM NOT DONE
+
+fn longest<'a>(x: &'a str, y: &'a str) -> &'a str {
+    if x.len() > y.len() {
+        x
+    } else {
+        y
+    }
+}
+
+fn main() {
+    let string1 = String::from("long string is long");
+    let result;
+    {
+        let string2 = String::from("xyz");
+        result = longest(string1.as_str(), string2.as_str());
+    }
+    println!("The longest string is {}", result);
+}