std: Fix IntoIter::as_mut_slice's signature

This was intended to require `&mut self`, not `&self`, otherwise it's unsound!

Closes rust-lang#39465

Author: alexcrichton

src/libcollections/vec.rs

@@ -1929,7 +1929,7 @@ impl<T> IntoIter<T> {
     /// assert_eq!(into_iter.next().unwrap(), 'z');
     /// ```
     #[stable(feature = "vec_into_iter_as_slice", since = "1.15.0")]
-    pub fn as_mut_slice(&self) -> &mut [T] {
+    pub fn as_mut_slice(&mut self) -> &mut [T] {
         unsafe {
             slice::from_raw_parts_mut(self.ptr as *mut T, self.len())
         }