Add solutions to bins

solutions/00_intro/intro1.rs

@@ -1,2 +1,5 @@
-// The exercise `intro1` only requires entering `n` in the terminal to go to the next exercise.
-// It is just an introduction to how Rustlings works.
+fn main() {
+    // Congratulations, you finished the first exercise 🎉
+    // As an introduction to Rustlings, the first exercise only required
+    // entering `n` in the terminal to go to the next exercise.
+}