refbox: add Debug impl

[jmesmon]

Allows #[derive(Debug)] to work with things containing RefBox.

Not sure adding impls like this is sustainable.

I used to be able to do

impl<'a> fmt::Debug for bincode::RefBox<'a, MyLocalType> {
    fn fmt(&self, fmt: &mut fmt::Formatter) -> Result<(), fmt::Error> {
        write!(fmt, "RefBox({:?})", *self)
    }
}

Locally, but recent rust has forbidden this.

[TyOverby]

Not implementing Debug was a mistake on my part. Thanks!