Logging glue dumps core on classes w/ destructors and boxes

[msullivan]

struct thing {
    x: {a: @int};
    new (x: {a: @int}) { self.x = x; }
    drop { }
}

fn main() {
    let x = thing({a: @10});
    log(error, x);
}

will dump core.

[catamorphism]

Dup of #2834