feat: add lifetimes exercises

exercises/lifetimes/lifetimes1.rs

@@ -0,0 +1,26 @@
+// lifetimes1.rs
+//
+// The Rust compiler needs to know how to check whether supplied references are
+// valid, so that it can let the programmer know if a reference is at risk
+// of going out of scope before it is used. Remember, references are borrows
+// and do not own their own data. What if their owner goes out of scope?
+//
+// Execute `rustlings hint lifetimes1` or use the `hint` watch subcommand for a hint.
+
+// I AM NOT DONE
+
+fn longest(x: &str, y: &str) -> &str {
+    if x.len() > y.len() {
+        x
+    } else {
+        y
+    }
+}
+
+fn main() {
+    let string1 = String::from("abcd");
+    let string2 = "xyz";
+
+    let result = longest(string1.as_str(), string2);
+    println!("The longest string is {}", result);
+}