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/03_if/if1.rs

@@ -0,0 +1,33 @@
+fn bigger(a: i32, b: i32) -> i32 {
+    // TODO: Complete this function to return the bigger number!
+    // If both numbers are equal, any of them can be returned.
+    // Do not use:
+    // - another function call
+    // - additional variables
+    if a > b { a } else { b }
+}
+
+fn main() {
+    // You can optionally experiment here.
+}
+
+// Don't mind this for now :)
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn ten_is_bigger_than_eight() {
+        assert_eq!(10, bigger(10, 8));
+    }
+
+    #[test]
+    fn fortytwo_is_bigger_than_thirtytwo() {
+        assert_eq!(42, bigger(32, 42));
+    }
+
+    #[test]
+    fn equal_numbers() {
+        assert_eq!(42, bigger(42, 42));
+    }
+}