Safeguard for Ticker internal storage that may be changed during callback execution

[mcspr]

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.

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() (since why not)

Ticker ticker;
void repeat() {
    ticker.once(1.0f, repeat);
    puts("tick");
}

void setup() {
    repeat();
}

Using temporary variable in the static_callback func and returning earlier should solve both cases.