Update next() and size_hint() for MutSpliterIterator

Update the next() method to just return self.v in the case that we've reached
the last element that the iterator will yield. This produces equivalent
behavior as before, but without the cost of updating the field.

Update the size_hint() method to return a better hint now that rust-lang#9629 is fixed.

src/libstd/vec.rs

@@ -2528,13 +2528,13 @@ impl<'a, T> Iterator<&'a mut [T]> for MutSplitIterator<'a, T> {
 
     #[inline]
     fn size_hint(&self) -> (uint, Option<uint>) {
-        if self.finished { return (0, Some(0)) }
-
-        // if the predicate doesn't match anything, we yield one slice
-        // if it matches every element, we yield len+1 empty slices.
-        // FIXME #9629
-        //(1, Some(self.v.len() + 1))
-        (1, None)
+        if self.finished {
+            (0, Some(0))
+        } else {
+            // if the predicate doesn't match anything, we yield one slice
+            // if it matches every element, we yield len+1 empty slices.
+            (1, Some(self.v.len() + 1))
+        }
     }
 }
 
@@ -2547,10 +2547,7 @@ impl<'a, T> DoubleEndedIterator<&'a mut [T]> for MutSplitIterator<'a, T> {
             None => {
                 self.finished = true;
                 let tmp = util::replace(&mut self.v, &mut []);
-                let len = tmp.len();
-                let (head, tail) = tmp.mut_split_at(len);
-                self.v = tail;
-                Some(head)
+                Some(tmp)
             }
             Some(idx) => {
                 let tmp = util::replace(&mut self.v, &mut []);