Replace hashbrown with ahash

hashbrown is already used in the standard library, but we want the
improved performance of the different hash algorithm.
Using ahash directly conveys this intent more clearly.

src/app_state.rs

@@ -1,3 +1,4 @@
+use ahash::{HashSet, HashSetExt};
 use anyhow::{bail, Context, Error, Result};
 use std::{
     fs::{self, File},
@@ -69,7 +70,7 @@ impl AppState {
             return StateFileStatus::NotRead;
         }
 
-        let mut done_exercises = hashbrown::HashSet::with_capacity(self.exercises.len());
+        let mut done_exercises = HashSet::with_capacity(self.exercises.len());
 
         for done_exerise_name in lines {
             if done_exerise_name.is_empty() {