feat: Index exercises by name

BREAKING CHANGE: This changes the way you use `rustlings run` by now
requiring an abridged form of the previous filename, e.g:

`rustlings run exercises/if/if1.rs` becomes
`rustlings run if1`

src/exercise.rs

@@ -24,6 +24,7 @@ pub struct ExerciseList {
 
 #[derive(Deserialize)]
 pub struct Exercise {
+    pub name: String,
     pub path: PathBuf,
     pub mode: Mode,
 }
@@ -70,6 +71,7 @@ mod test {
     fn test_clean() {
         File::create(&temp_file()).unwrap();
         let exercise = Exercise {
+            name: String::from("example"),
             path: PathBuf::from("example.rs"),
             mode: Mode::Test,
         };