Add some exercises about Strings and &strs!

strings/strings2.rs

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+// Make me compile without changing the function signature! Scroll down for hints :)
+
+fn main() {
+    let guess1 = "blue".to_string(); // Try not changing this line :)
+    let correct = guess_favorite_color(guess1);
+    if correct {
+        println!("You guessed correctly!");
+    } else {
+        println!("Nope, that's not it.");
+    }
+}
+
+fn guess_favorite_color(attempt: &str) -> bool {
+    attempt == "green"
+}
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+// Yes, it would be really easy to fix this by just changing the value bound to `guess1` to be a
+// string slice instead of a `String`, wouldn't it?? There is a way to add one character to line
+// 5, though, that will coerce the `String` into a string slice.