chapter 20 - remove FnBox workaround

[joekyo]

As in Rust 1.35.0, FnBox is no loger needed.
https://blog.rust-lang.org/2019/05/23/Rust-1.35.0.html

It is perfectly OK to call job(); now

match message {
    Message::NewJob(job) => {
        job();
    }

Is there any plan to remove the FnBox workaround?

[steveklabnik]

Yep, this is an important thing to do!

[joekyo]

Close as it has been fixed in #1906