refactor: exercise evaluation

Exercise evaluation (compilation + execution) now uses Results
Success/failure messages are standardized

src/main.rs

@@ -15,6 +15,9 @@ use std::sync::{Arc, Mutex};
 use std::thread;
 use std::time::Duration;
 
+#[macro_use]
+mod ui;
+
 mod exercise;
 mod run;
 mod verify;