fix `dyn std::ops::FnMut()` cannot be sent between threads safely #20

[alopatindev]

[rakshith-ravi]

@lholden can we have this merged and published to crates.io please?

[bellackn]

Seconding that @alopatindev - awesome crate, but this PR would make it even more awesome, @lholden! :) Tested it and this fixes it in my case:

    thread::spawn(move || {
        loop {
            schedule.tick();
            thread::sleep(std::time::Duration::from_secs(60));
        }
    });

[BlackDex]

This PR is implemented in https://crates.io/crates/job_scheduler_ng (with some minor changes).