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/modules3.rs

@@ -0,0 +1,13 @@
+// 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!"),
+    }
+}