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/12_options/options1.rs

@@ -0,0 +1,43 @@
+// This function returns how much ice cream there is left in the fridge.
+// If it's before 22:00 (24-hour system), then 5 scoops are left. At 22:00,
+// someone eats it all, so no ice cream is left (value 0). Return `None` if
+// `hour_of_day` is higher than 23.
+fn maybe_ice_cream(hour_of_day: u16) -> Option<u16> {
+    // TODO: Complete the function body.
+    if hour_of_day > 23 {
+        None
+    } else if hour_of_day >= 22 {
+        Some(0)
+    } else {
+        Some(5)
+    }
+}
+
+fn main() {
+    // You can optionally experiment here.
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn raw_value() {
+        // TODO: Fix this test. How do you get the value contained in the
+        // Option?
+        let ice_creams = maybe_ice_cream(12);
+
+        assert_eq!(ice_creams, Some(5)); // Don't change this line.
+    }
+
+    #[test]
+    fn check_ice_cream() {
+        assert_eq!(maybe_ice_cream(0), Some(5));
+        assert_eq!(maybe_ice_cream(9), Some(5));
+        assert_eq!(maybe_ice_cream(18), Some(5));
+        assert_eq!(maybe_ice_cream(22), Some(0));
+        assert_eq!(maybe_ice_cream(23), Some(0));
+        assert_eq!(maybe_ice_cream(24), None);
+        assert_eq!(maybe_ice_cream(25), None);
+    }
+}