Implement support for serializing Box<[u8]> and other pointers of [u8]

[reem]

This should work basically the same as with Vec<u8>.

[Parakleta]

Related to this, also add the reference generic:

impl<'a, T: ?Sized + ToJson> ToJson for &'a T {
    fn to_json(&self) -> Json { (**self).to_json() }
}

[alexcrichton]

I'm going to close this now that this crate is deprecated in favor of serde. We're discontinuing feature development in rustc-serialize but will still continue to merge bug fixes if they arise.