rewrite sort as sort_unstable where applicable in exercise examples (#1015)

Author: coriolinus

exercises/anagram/example.rs

@@ -2,7 +2,7 @@ use std::collections::HashSet;
 
 fn sort(word: &str) -> String {
     let mut sorted: Vec<char> = word.chars().collect();
-    sorted.sort();
+    sorted.sort_unstable();
     sorted.into_iter().collect()
 }
 

exercises/grade-school/example.rs

@@ -14,12 +14,12 @@ impl School {
     pub fn add(&mut self, grade: u32, student: &str) {
         let entry = self.grades.entry(grade).or_insert_with(Vec::new);
         entry.push(student.to_string());
-        entry.sort();
+        entry.sort_unstable();
     }
 
     pub fn grades(&self) -> Vec<u32> {
         let mut s = self.grades.keys().cloned().collect::<Vec<u32>>();
-        s.sort();
+        s.sort_unstable();
         s
     }
 

exercises/high-scores/example.rs

@@ -22,7 +22,7 @@ impl<'a> HighScores<'a> {
 
     pub fn personal_top_three(&self) -> Vec<u32> {
         let mut sorted = self.scores.to_vec();
-        sorted.sort();
+        sorted.sort_unstable();
         sorted.reverse();
         sorted.truncate(3);
         sorted