Implement time::repeat and simplify Subscription::run_with

[hecrj]

Let's say you have an async function to obtain the current weather:

async fn fetch_weather() -> Result<Weather, Error> {
    // ...
}

The new time::repeat function can be leveraged to turn this async function into a Subscription that runs it at a set interval:

use iced::time;

enum Message {
    WeatherFetched(Result<Weather, Error>),
}

fn subscription(state: &State) -> Subscription<Message> {
    time::repeat(fetch_weather, time::Duration::from_secs(5))
        .map(Message::WeatherFetched)
}