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/22_clippy/clippy3.rs

@@ -0,0 +1,28 @@
+// Here are some more easy Clippy fixes so you can see its utility 📎
+// TODO: Fix all the Clippy lints.
+
+#[rustfmt::skip]
+#[allow(unused_variables, unused_assignments)]
+fn main() {
+    let my_option: Option<&str> = None;
+    // Assume that you don't know the value of `my_option`.
+    // In the case of `Some`, we want to print its value.
+    if let Some(val) = my_option {
+        println!("{}", val);
+    }
+
+    let my_arr = &[
+        -1, -2, -3,
+        -4, -5, -6
+    ];
+    println!("My array! Here it is: {my_arr:?}");
+
+    let my_empty_vec: Vec<i32> = vec![];
+    println!("This Vec is empty, see? {my_empty_vec:?}");
+
+    let mut value_a = 45;
+    let mut value_b = 66;
+    // Let's swap these two!
+    std::mem::swap(&mut value_a, &mut value_b);
+    println!("value a: {value_a}; value b: {value_b}");
+}