safety: conform to stacked borrows

When using 'get_unchecked' twice where one is a mutable borrow, it ends
up creating UB under the "stacked borrows" model. Which isn't adopted
yet. Still, it seems likely that it will? So we fix it by deriving both
pointers to 'ptr::copy' from the same 'get_unchecked_mut' call.

Closes #121

src/ext_slice.rs

@@ -3063,11 +3063,8 @@ pub trait ByteSlice: Sealed {
         // Finally, we are only dealing with u8 data, which is Copy, which
         // means we can copy without worrying about ownership/destructors.
         unsafe {
-            ptr::copy(
-                self.as_bytes().get_unchecked(src_start),
-                self.as_bytes_mut().get_unchecked_mut(dest),
-                count,
-            );
+            let ptr = self.as_bytes_mut().as_mut_ptr();
+            ptr::copy(ptr.add(src_start), ptr.add(dest), count);
         }
     }
 }