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/07_structs/structs1.rs

@@ -0,0 +1,50 @@
+struct ColorRegularStruct {
+    // TODO: Add the fields that the test `regular_structs` expects.
+    // What types should the fields have? What are the minimum and maximum values for RGB colors?
+    red: u8,
+    green: u8,
+    blue: u8,
+}
+
+struct ColorTupleStruct(u8, u8, u8);
+
+#[derive(Debug)]
+struct UnitStruct;
+
+fn main() {
+    // You can optionally experiment here.
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn regular_structs() {
+        // TODO: Instantiate a regular struct.
+        let green = ColorRegularStruct { red: 0, green: 255, blue: 0 };
+
+        assert_eq!(green.red, 0);
+        assert_eq!(green.green, 255);
+        assert_eq!(green.blue, 0);
+    }
+
+    #[test]
+    fn tuple_structs() {
+        // TODO: Instantiate a tuple struct.
+        let green = ColorTupleStruct(0, 255, 0);
+
+        assert_eq!(green.0, 0);
+        assert_eq!(green.1, 255);
+        assert_eq!(green.2, 0);
+    }
+
+    #[test]
+    fn unit_structs() {
+        // TODO: Instantiate a unit struct.
+        let unit_struct = UnitStruct;
+        let message = format!("{unit_struct:?}s are fun!");
+
+        assert_eq!(message, "UnitStructs are fun!");
+    }
+}