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/quizzes/quiz2.rs

@@ -0,0 +1,77 @@
+// This is a quiz for the following sections:
+// - Strings
+// - Vecs
+// - Move semantics
+// - Modules
+// - Enums
+//
+// Let's build a little machine in the form of a function. As input, we're going
+// to give a list of strings and commands. These commands determine what action
+// is going to be applied to the string. It can either be:
+// - Uppercase the string
+// - Trim the string
+// - Append "bar" to the string a specified amount of times
+//
+// The exact form of this will be:
+// - The input is going to be a Vector of 2-length tuples,
+//   the first element is the string, the second one is the command.
+// - The output element is going to be a vector of strings.
+
+enum Command {
+    Uppercase,
+    Trim,
+    Append(usize),
+}
+
+mod my_module {
+    use super::Command;
+
+    // TODO: Complete the function as described above.
+    pub fn transformer(input: Vec<(String, Command)>) -> Vec<String> {
+        input.iter().map(|(s, cmd)| {
+            match cmd {
+                Command::Uppercase => s.to_uppercase(),
+                Command::Trim => s.trim().to_string(),
+                Command::Append(n) => {
+                    let mut result = s.clone();
+                    for _ in 0..*n {
+                        result.push_str("bar");
+                    }
+                    result
+                }
+            }
+        }).collect()
+    }
+}
+
+fn main() {
+    // You can optionally experiment here.
+}
+
+#[cfg(test)]
+mod tests {
+    // TODO: What do we need to import to have `transformer` in scope?
+    use super::my_module::transformer;
+    use super::Command;
+
+    #[test]
+    fn it_works() {
+        let input = vec![
+            ("hello".to_string(), Command::Uppercase),
+            (" all roads lead to rome! ".to_string(), Command::Trim),
+            ("foo".to_string(), Command::Append(1)),
+            ("bar".to_string(), Command::Append(5)),
+        ];
+        let output = transformer(input);
+
+        assert_eq!(
+            output,
+            [
+                "HELLO",
+                "all roads lead to rome!",
+                "foobar",
+                "barbarbarbarbarbar",
+            ]
+        );
+    }
+}