No more FnBox - The Edition Guide

# No more FnBox

The book used to have this code in Chapter 20, section 2:

    #![allow(unused)]
    fn main() {
    trait FnBox {
        fn call_box(self: Box<Self>);
    }

    impl<F: FnOnce()> FnBox for F {
        fn call_box(self: Box<F>) {
            (*self)()
        }
    }

    type Job = Box<dyn FnBox + Send + 'static>;
    }

Here, we define a new trait called `FnBox`, and then implement it for all `FnOnce` closures. All the implementation does is call the closure. These sorts of hacks were needed because a `Box<dyn FnOnce>` didn't implement `FnOnce`. This was true for all three posibilities:

- `Box<dyn Fn>` and `Fn`
- `Box<dyn FnMut>` and `FnMut`
- `Box<dyn FnOnce>` and `FnOnce`

However, as of Rust 1.35, these traits are implemented for these types, and so the `FnBox` trick is no longer required. In the latest version of the book, the `Job` type looks like this:

    #![allow(unused)]
    fn main() {
    type Job = Box<dyn FnOnce() + Send + 'static>;
    }

No need for all that other code.