Note that the workaround to force `change_x` out of scope in the section 'Implications of “Closures are Structs”' is no longer necessary with non-lexical lifeti

Note that the workaround to force `change_x` out of scope in the section 'Implications of “Closures are Structs”' is no longer necessary with non-lexical lifetimes. So Rust closures are (somewhat) less hard now :)