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/19_smart_pointers/box1.rs

@@ -0,0 +1,50 @@
+// At compile time, Rust needs to know how much space a type takes up. This
+// becomes problematic for recursive types, where a value can have as part of
+// itself another value of the same type. To get around the issue, we can use a
+// `Box` - a smart pointer used to store data on the heap, which also allows us
+// to wrap a recursive type.
+//
+// The recursive type we're implementing in this exercise is the "cons list", a
+// data structure frequently found in functional programming languages. Each
+// item in a cons list contains two elements: The value of the current item and
+// the next item. The last item is a value called `Nil`.
+
+// TODO: Use a `Box` in the enum definition to make the code compile.
+#[derive(PartialEq, Debug)]
+enum List {
+    Cons(i32, Box<List>),
+    Nil,
+}
+
+// TODO: Create an empty cons list.
+fn create_empty_list() -> List {
+    List::Nil
+}
+
+// TODO: Create a non-empty cons list.
+fn create_non_empty_list() -> List {
+    List::Cons(1, Box::new(List::Cons(2, Box::new(List::Nil))))
+}
+
+fn main() {
+    println!("This is an empty cons list: {:?}", create_empty_list());
+    println!(
+        "This is a non-empty cons list: {:?}",
+        create_non_empty_list(),
+    );
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_create_empty_list() {
+        assert_eq!(create_empty_list(), List::Nil);
+    }
+
+    #[test]
+    fn test_create_non_empty_list() {
+        assert_ne!(create_empty_list(), create_non_empty_list());
+    }
+}