Add initial solution files for Rustlings exercises and quizzes

- Created empty solution files for various exercises in strings, modules, hashmaps, options, error handling, generics, traits, lifetimes, tests, iterators, smart pointers, threads, macros, clippy, conversions, and quizzes.
- Each solution file contains a main function with a comment indicating that it will be automatically filled after completing the exercise.
- Added a README.md file to provide information about the solutions and their purpose.

exercises/18_iterators/iterators1.rs

@@ -0,0 +1,25 @@
+// When performing operations on elements within a collection, iterators are
+// essential. This module helps you get familiar with the structure of using an
+// iterator and how to go through elements within an iterable collection.
+
+fn main() {
+    // You can optionally experiment here.
+}
+
+#[cfg(test)]
+mod tests {
+    #[test]
+    fn iterators() {
+        let my_fav_fruits = ["banana", "custard apple", "avocado", "peach", "raspberry"];
+
+        // TODO: Create an iterator over the array.
+        let mut fav_fruits_iterator = my_fav_fruits.iter();
+
+        assert_eq!(fav_fruits_iterator.next(), Some(&"banana"));
+        assert_eq!(fav_fruits_iterator.next(), Some(&"custard apple")); // TODO: Replace `todo!()`
+        assert_eq!(fav_fruits_iterator.next(), Some(&"avocado"));
+        assert_eq!(fav_fruits_iterator.next(), Some(&"peach")); // TODO: Replace `todo!()`
+        assert_eq!(fav_fruits_iterator.next(), Some(&"raspberry"));
+        assert_eq!(fav_fruits_iterator.next(), None); // TODO: Replace `todo!()`
+    }
+}