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/quiz1.rs

@@ -0,0 +1,37 @@
+// This is a quiz for the following sections:
+// - Variables
+// - Functions
+// - If
+//
+// Mary is buying apples. The price of an apple is calculated as follows:
+// - An apple costs 2 rustbucks.
+// - However, if Mary buys more than 40 apples, the price of each apple in the
+// entire order is reduced to only 1 rustbuck!
+
+// TODO: Write a function that calculates the price of an order of apples given
+// the quantity bought.
+fn calculate_price_of_apples(quantity: i32) -> i32 {
+    if quantity > 40 {
+        quantity
+    } else {
+        quantity * 2
+    }
+}
+
+fn main() {
+    // You can optionally experiment here.
+}
+
+// Don't change the tests!
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn verify_test() {
+        assert_eq!(calculate_price_of_apples(35), 70);
+        assert_eq!(calculate_price_of_apples(40), 80);
+        assert_eq!(calculate_price_of_apples(41), 41);
+        assert_eq!(calculate_price_of_apples(65), 65);
+    }
+}