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/08_enums/enums1.rs

@@ -0,0 +1,17 @@
+#[derive(Debug)]
+enum Message {
+    // TODO: Define a few types of messages as used below.
+    Resize,
+    Move,
+    Echo,
+    ChangeColor,
+    Quit,
+}
+
+fn main() {
+    println!("{:?}", Message::Resize);
+    println!("{:?}", Message::Move);
+    println!("{:?}", Message::Echo);
+    println!("{:?}", Message::ChangeColor);
+    println!("{:?}", Message::Quit);
+}