Mainly, we protect std::function and variant itself from undefined
behaviour when the underlying object is destroyed and then re-created
with something else inside of it.

For example, every captured object would be destroyed.
```cpp
Ticker ticker;
void oops(Job job) {
    ticker.once(1.0f,
        [job = std::move(job)]() {
            ticker.once(job.next(), do_something_else);
            job.work(); // ~Job() just happened!
        });
}
```

Another important case is repeating through once()
```cpp
Ticker ticker;
void repeat() {
    ticker.once(1.0f, repeat);
    puts("tick");
}

void setup() {
    repeat();
}
```

Temporarily moving callback object into the function and returning
earlier should solve both cases.