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/10_modules/README.md

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+# Modules
+
+In this section we'll give you an introduction to Rust's module system.
+
+## Further information
+
+- [The Module System](https://doc.rust-lang.org/book/ch07-00-managing-growing-projects-with-packages-crates-and-modules.html)

exercises/10_modules/modules1.rs

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+// TODO: Fix the compiler error about calling a private function.
+mod sausage_factory {
+    // Don't let anybody outside of this module see this!
+    fn get_secret_recipe() -> String {
+        String::from("Ginger")
+    }
+
+    pub fn make_sausage() {
+        get_secret_recipe();
+        println!("sausage!");
+    }
+}
+
+fn main() {
+    sausage_factory::make_sausage();
+}

exercises/10_modules/modules2.rs

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+// You can bring module paths into scopes and provide new names for them with
+// the `use` and `as` keywords.
+
+mod delicious_snacks {
+    // TODO: Add the following two `use` statements after fixing them.
+    pub use self::fruits::PEAR as fruit;
+    pub use self::veggies::CUCUMBER as veggie;
+
+    mod fruits {
+        pub const PEAR: &str = "Pear";
+        pub const APPLE: &str = "Apple";
+    }
+
+    mod veggies {
+        pub const CUCUMBER: &str = "Cucumber";
+        pub const CARROT: &str = "Carrot";
+    }
+}
+
+fn main() {
+    println!(
+        "favorite snacks: {} and {}",
+        delicious_snacks::fruit,
+        delicious_snacks::veggie,
+    );
+}

exercises/10_modules/modules3.rs

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+// You can use the `use` keyword to bring module paths from modules from
+// anywhere and especially from the standard library into your scope.
+
+// TODO: Bring `SystemTime` and `UNIX_EPOCH` from the `std::time` module into
+// your scope. Bonus style points if you can do it with one line!
+use std::time::{SystemTime, UNIX_EPOCH};
+
+fn main() {
+    match SystemTime::now().duration_since(UNIX_EPOCH) {
+        Ok(n) => println!("1970-01-01 00:00:00 UTC was {} seconds ago!", n.as_secs()),
+        Err(_) => panic!("SystemTime before UNIX EPOCH!"),
+    }
+}