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/15_traits/traits2.rs

@@ -0,0 +1,29 @@
+trait AppendBar {
+    fn append_bar(self) -> Self;
+}
+
+// TODO: Implement the trait `AppendBar` for a vector of strings.
+// `append_bar` should push the string "Bar" into the vector.
+impl AppendBar for Vec<String> {
+    fn append_bar(self) -> Self {
+        let mut result = self;
+        result.push(String::from("Bar"));
+        result
+    }
+}
+
+fn main() {
+    // You can optionally experiment here.
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn is_vec_pop_eq_bar() {
+        let mut foo = vec![String::from("Foo")].append_bar();
+        assert_eq!(foo.pop().unwrap(), "Bar");
+        assert_eq!(foo.pop().unwrap(), "Foo");
+    }
+}