Use for_each in Iterator::partition

We already use this for `unzip`, but `partition` is not much different.

src/libcore/iter/traits/iterator.rs

@@ -1495,13 +1495,13 @@ pub trait Iterator {
         let mut left: B = Default::default();
         let mut right: B = Default::default();
 
-        for x in self {
+        self.for_each(|x| {
             if f(&x) {
                 left.extend(Some(x))
             } else {
                 right.extend(Some(x))
             }
-        }
+        });
 
         (left, right)
     }