Use `std::future::pending` for infinite sleep

What it does

Whenever you write tokio::time::sleep(Duration::MAX), the lint should suggest that you replace this with std::future::pending().

Ideally the lint would also apply to sleep calls from other runtimes.

Advantage

• Improves efficiency by not storing a timer.
• Does not depend on a specific runtime.
• The lint will teach people that std::future::pending() exists.

Drawbacks

Perhaps this lint is too specific?

Example

#[tokio::main]
async fn main() {
    tokio::spawn(my_bg_task());
    // Don't return from main, since that kills everything.
    tokio::time::sleep(Duration::MAX).await;
}

Could be written as:

#[tokio::main]
async fn main() {
    tokio::spawn(my_bg_task());
    // Don't return from main, since that kills everything.
    std::future::pending().await;
}

Start from the issue's Tokio example and the requested replacement with std::future::pending(). Determine how the lint should recognize Duration::MAX sleeps, including whether other runtimes are in scope; done means the intended sleep calls produce the suggested replacement without incorrectly flagging other uses.