Function show_notes::e005::doubler_closure_factory

pub fn doubler_closure_factory() -> Box<dyn Fn(i32) -> i32>

Creates a closure which doubles an integer.

Compare with the function above: this is basically identical to doubler_factory, the only difference being that in this case, we return a closure rather than a standalone function. That has substantial ramifications for using closures and named functions, of course; refer to the discussion in e004 for details