Apply suggestions from code review

Co-authored-by: Eliza Weisman <eliza@buoyant.io>

tokio/src/sync/mutex.rs

@@ -2,9 +2,7 @@ use crate::sync::batch_semaphore as semaphore;
 
 use std::cell::UnsafeCell;
 use std::error::Error;
-use std::fmt;
-use std::marker;
-use std::mem;
+use std::{fmt, marker, mem};
 use std::ops::{Deref, DerefMut};
 use std::sync::Arc;
 
@@ -135,7 +133,7 @@ pub struct MutexGuard<'a, T: ?Sized> {
     s: &'a semaphore::Semaphore,
     data: *mut T,
     // Needed to tell the borrow checker that we are holding a `&mut T`
-    marker: marker::PhantomData<&'a mut T>,
+    _marker: marker::PhantomData<&'a mut T>,
 }
 
 /// An owned handle to a held `Mutex`.