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.

solutions/15_traits/traits1.rs

@@ -0,0 +1,4 @@
+fn main() {
+    // DON'T EDIT THIS SOLUTION FILE!
+    // It will be automatically filled after you finish the exercise.
+}

solutions/15_traits/traits2.rs

@@ -0,0 +1,4 @@
+fn main() {
+    // DON'T EDIT THIS SOLUTION FILE!
+    // It will be automatically filled after you finish the exercise.
+}

solutions/15_traits/traits3.rs

@@ -0,0 +1,4 @@
+fn main() {
+    // DON'T EDIT THIS SOLUTION FILE!
+    // It will be automatically filled after you finish the exercise.
+}

solutions/15_traits/traits4.rs

@@ -0,0 +1,35 @@
+trait Licensed {
+    fn licensing_info(&self) -> String {
+        "Default license".to_string()
+    }
+}
+
+struct SomeSoftware;
+struct OtherSoftware;
+
+impl Licensed for SomeSoftware {}
+impl Licensed for OtherSoftware {}
+
+fn compare_license_types(software1: impl Licensed, software2: impl Licensed) -> bool {
+    //                              ^^^^^^^^^^^^^             ^^^^^^^^^^^^^
+    software1.licensing_info() == software2.licensing_info()
+}
+
+fn main() {
+    // You can optionally experiment here.
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn compare_license_information() {
+        assert!(compare_license_types(SomeSoftware, OtherSoftware));
+    }
+
+    #[test]
+    fn compare_license_information_backwards() {
+        assert!(compare_license_types(OtherSoftware, SomeSoftware));
+    }
+}

solutions/15_traits/traits5.rs

@@ -0,0 +1,39 @@
+trait SomeTrait {
+    fn some_function(&self) -> bool {
+        true
+    }
+}
+
+trait OtherTrait {
+    fn other_function(&self) -> bool {
+        true
+    }
+}
+
+struct SomeStruct;
+impl SomeTrait for SomeStruct {}
+impl OtherTrait for SomeStruct {}
+
+struct OtherStruct;
+impl SomeTrait for OtherStruct {}
+impl OtherTrait for OtherStruct {}
+
+fn some_func(item: impl SomeTrait + OtherTrait) -> bool {
+    //             ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+    item.some_function() && item.other_function()
+}
+
+fn main() {
+    // You can optionally experiment here.
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_some_func() {
+        assert!(some_func(SomeStruct));
+        assert!(some_func(OtherStruct));
+    }
+}