Revert "Slightly refactor VecDeque implementation"

This reverts commit 6ce76ac.

Author: alexcrichton

src/liballoc/collections/vec_deque.rs

@@ -1024,7 +1024,7 @@ impl<T> VecDeque<T> {
             iter: Iter {
                 tail: drain_tail,
                 head: drain_head,
-                ring: unsafe { self.buffer_as_slice() },
+                ring: unsafe { self.buffer_as_mut_slice() },
             },
         }
     }
@@ -2593,8 +2593,8 @@ impl<T> From<VecDeque<T>> for Vec<T> {
                         let mut right_offset = 0;
                         for i in left_edge..right_edge {
                             right_offset = (i - left_edge) % (cap - right_edge);
-                            let src = right_edge + right_offset;
-                            ptr::swap(buf.add(i), buf.add(src));
+                            let src: isize = (right_edge + right_offset) as isize;
+                            ptr::swap(buf.add(i), buf.offset(src));
                         }
                         let n_ops = right_edge - left_edge;
                         left_edge += n_ops;