Relaxed runner type from Fn to FnOnce

[B-head]

Objective

Relax unnecessary type restrictions on App.runner function.

Solution

Changed the type of App.runner from Fn(App) to FnOnce(App).

[github-actions]

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[alice-i-cecile]

This makes sense, although I would expect that almost all use cases meet the tighter bounds.

What motivated this?

[alice-i-cecile]

Aha, the linked PR provides a good motivation :D

[alice-i-cecile]

@nakedible: does this PR look right to you as well?

[nakedible]

This is correct and adheres exactly to documentation of set_runner which says:

The runner function run_fn is called only once by [App::run].

Also, the code that actually calls this:

        let runner = std::mem::replace(&mut app.runner, Box::new(run_once));
        (runner)(app);