Go back to Some instead of transmuting to it.

This adds a few more statements to `next`, but optimizes better in the loops (saving 2 blocks in `forward_loop`, for example)

Author: scottmcm

Diff for: core/src/slice/iter/macros.rs

@@ -173,7 +173,7 @@ macro_rules! iterator {
                         }
                         self.ptr = ptr.add(1);
                     }
-                    crate::intrinsics::transmute::<NonNull<T>, Option<$elem>>(ptr)
+                    Some({ptr}.$into_ref())
                 }
             }