grade-school, bowling: Remove unnecessary mut (#346)

`iter_mut()` and `or_insert()` yields mutable references, the variables 
they bind to does not need to be mutable. 

Previously, because of rust-lang/rust#30280
this was not considered a warning.

The fix is in rust-lang/rust#43582
This means on recently nightlies this warning is emitted.

Author: petertseng

exercises/bowling/example.rs

@@ -104,7 +104,7 @@ impl BowlingGame {
                 return Err("Game Finished. No more rolls.");
             }
 
-            for mut frame in self.frames.iter_mut() {
+            for frame in self.frames.iter_mut() {
                 frame.add_roll(pins)
             }
 

exercises/grade-school/example.rs

@@ -10,7 +10,7 @@ impl School {
     }
 
     pub fn add(&mut self, grade: u32, student: &str) {
-        let mut entry = self.grades.entry(grade).or_insert(Vec::new());
+        let entry = self.grades.entry(grade).or_insert(Vec::new());
         entry.push(student.to_string());
         entry.sort();
     }