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/if2.rs

@@ -0,0 +1,39 @@
+// TODO: Fix the compiler error on this function.
+fn picky_eater(food: &str) -> &str {
+    if food == "strawberry" {
+        "Yummy!"
+    } else if food == "potato" {
+        "I guess I can eat that."
+    } else {
+        "No thanks!"
+    }
+}
+
+fn main() {
+    // You can optionally experiment here.
+}
+
+// TODO: Read the tests to understand the desired behavior.
+// Make all tests pass without changing them.
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn yummy_food() {
+        // This means that calling `picky_eater` with the argument "strawberry" should return "Yummy!".
+        assert_eq!(picky_eater("strawberry"), "Yummy!");
+    }
+
+    #[test]
+    fn neutral_food() {
+        assert_eq!(picky_eater("potato"), "I guess I can eat that.");
+    }
+
+    #[test]
+    fn default_disliked_food() {
+        assert_eq!(picky_eater("broccoli"), "No thanks!");
+        assert_eq!(picky_eater("gummy bears"), "No thanks!");
+        assert_eq!(picky_eater("literally anything"), "No thanks!");
+    }
+}