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/20_threads/README.md

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+# Threads
+
+In most current operating systems, an executed program's code is run in a process, and the operating system manages multiple processes at once.
+Within your program, you can also have independent parts that run simultaneously. The features that run these independent parts are called threads.
+
+## Further information
+
+- [Dining Philosophers example](https://doc.rust-lang.org/1.4.0/book/dining-philosophers.html)
+- [Using Threads to Run Code Simultaneously](https://doc.rust-lang.org/book/ch16-01-threads.html)
+- [Using Message Passing to Transfer Data Between Threads](https://doc.rust-lang.org/book/ch16-02-message-passing.html)

exercises/20_threads/threads1.rs

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+// This program spawns multiple threads that each runs for at least 250ms, and
+// each thread returns how much time it took to complete. The program should
+// wait until all the spawned threads have finished and should collect their
+// return values into a vector.
+
+use std::{
+    thread,
+    time::{Duration, Instant},
+};
+
+fn main() {
+    let mut handles = Vec::new();
+    for i in 0..10 {
+        let handle = thread::spawn(move || {
+            let start = Instant::now();
+            thread::sleep(Duration::from_millis(250));
+            println!("Thread {i} done");
+            start.elapsed().as_millis()
+        });
+        handles.push(handle);
+    }
+
+    let mut results = Vec::new();
+    for handle in handles {
+        // TODO: Collect the results of all threads into the `results` vector.
+        // Use the `JoinHandle` struct which is returned by `thread::spawn`.
+        results.push(handle.join().unwrap());
+    }
+
+    if results.len() != 10 {
+        panic!("Oh no! Some thread isn't done yet!");
+    }
+
+    println!();
+    for (i, result) in results.into_iter().enumerate() {
+        println!("Thread {i} took {result}ms");
+    }
+}

exercises/20_threads/threads2.rs

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+// Building on the last exercise, we want all of the threads to complete their
+// work. But this time, the spawned threads need to be in charge of updating a
+// shared value: `JobStatus.jobs_done`
+
+use std::{sync::{Arc, Mutex}, thread, time::Duration};
+
+struct JobStatus {
+    jobs_done: u32,
+}
+
+fn main() {
+    // TODO: `Arc` isn't enough if you want a **mutable** shared state.
+    let status = Arc::new(Mutex::new(JobStatus { jobs_done: 0 }));
+
+    let mut handles = Vec::new();
+    for _ in 0..10 {
+        let status_shared = Arc::clone(&status);
+        let handle = thread::spawn(move || {
+            thread::sleep(Duration::from_millis(250));
+
+            // TODO: You must take an action before you update a shared value.
+            status_shared.lock().unwrap().jobs_done += 1;
+        });
+        handles.push(handle);
+    }
+
+    // Waiting for all jobs to complete.
+    for handle in handles {
+        handle.join().unwrap();
+    }
+
+    // TODO: Print the value of `JobStatus.jobs_done`.
+    println!("Jobs done: {}", status.lock().unwrap().jobs_done);
+}

exercises/20_threads/threads3.rs

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+use std::{sync::mpsc, thread, time::Duration};
+
+struct Queue {
+    first_half: Vec<u32>,
+    second_half: Vec<u32>,
+}
+
+impl Queue {
+    fn new() -> Self {
+        Self {
+            first_half: vec![1, 2, 3, 4, 5],
+            second_half: vec![6, 7, 8, 9, 10],
+        }
+    }
+}
+
+fn send_tx(q: Queue, tx: mpsc::Sender<u32>) {
+    // TODO: We want to send `tx` to both threads. But currently, it is moved
+    // into the first thread. How could you solve this problem?
+    let tx2 = tx.clone();
+    thread::spawn(move || {
+        for val in q.first_half {
+            println!("Sending {val:?}");
+            tx.send(val).unwrap();
+            thread::sleep(Duration::from_millis(250));
+        }
+    });
+
+    thread::spawn(move || {
+        for val in q.second_half {
+            println!("Sending {val:?}");
+            tx2.send(val).unwrap();
+            thread::sleep(Duration::from_millis(250));
+        }
+    });
+}
+
+fn main() {
+    // You can optionally experiment here.
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn threads3() {
+        let (tx, rx) = mpsc::channel();
+        let queue = Queue::new();
+
+        send_tx(queue, tx);
+
+        let mut received = Vec::with_capacity(10);
+        for value in rx {
+            received.push(value);
+        }
+
+        received.sort();
+        assert_eq!(received, [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]);
+    }
+}