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/21_macros/README.md

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+# Macros
+
+Rust's macro system is very powerful, but also kind of difficult to wrap your
+head around. We're not going to teach you how to write your own fully-featured
+macros. Instead, we'll show you how to use and create them.
+
+If you'd like to learn more about writing your own macros, the
+[macrokata](https://github.com/tfpk/macrokata) project has a similar style
+of exercises to Rustlings, but is all about learning to write Macros.
+
+## Further information
+
+- [The Rust Book - Macros](https://doc.rust-lang.org/book/ch20-05-macros.html)
+- [The Little Book of Rust Macros](https://veykril.github.io/tlborm/)
+- [Rust by Example - macro_rules!](https://doc.rust-lang.org/rust-by-example/macros.html)