Vec's IntoIter should be Send/Sync when Vec is (fixes #21027)

src/libcollections/vec.rs

@@ -1511,6 +1511,9 @@ pub struct IntoIter<T> {
     end: *const T
 }
 
+unsafe impl<T: Send> Send for IntoIter<T> { }
+unsafe impl<T: Sync> Sync for IntoIter<T> { }
+
 impl<T> IntoIter<T> {
     #[inline]
     /// Drops all items that have not yet been moved and returns the empty vector.