Infinite recursion when traversing a cyclic data structure with repr

``` rust
struct A { a: Option<@mut A> }
fn main() {
  let a = @mut A { a: None }; let mut v = Some(a); a.a <-> v;
  io::println(fmt!("%?",a));
}
```

This causes the formatter to go into an infinite loop.

Nominating for maturity 3, since it's mostly likely a problem with the reflection api.
