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/README.md

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+# Smart Pointers
+
+In Rust, smart pointers are variables that contain an address in memory and reference some other data, but they also have additional metadata and capabilities.
+Smart pointers in Rust often own the data they point to, while references only borrow data.
+
+## Further Information
+
+- [Smart Pointers](https://doc.rust-lang.org/book/ch15-00-smart-pointers.html)
+- [Using Box to Point to Data on the Heap](https://doc.rust-lang.org/book/ch15-01-box.html)
+- [Rc\<T\>, the Reference Counted Smart Pointer](https://doc.rust-lang.org/book/ch15-04-rc.html)
+- [Shared-State Concurrency](https://doc.rust-lang.org/book/ch16-03-shared-state.html)
+- [Cow Documentation](https://doc.rust-lang.org/std/borrow/enum.Cow.html)