Add blob to the State::waitingForDownload variant

[rambip]

Using an enum for the Model of your app is probably more idiomatic, but working with it with the borrow checker is harder.

Here is what you could do for State::WaitingForDownload(blob)

fn give_me_blob(&mut State) -> Option<blob> {
    // FIXME: reset state only if in State ::WaitingForDownload
    let old_state = std::mem::replace(
        self.state,
        State::Downloading
        );
     match old_state {
          WaitingForDownload(blob) => Some(blob),
          _ => None
     }
}

Here is an exemple:

https://rust-unofficial.github.io/patterns/idioms/mem-replace.html