Allow running Task::local from any thread

[oblique]

This enables us to use Task::local from any thread, this is how smol 0.1 behavior was.
We achieve this by having a thread local LocalExecutor.

The following is now possible:

use smol::Task;

fn main() {
    smol::run(async {
        Task::spawn(async {
            Task::local(async {
                println!("{:?}", std::thread::current().id());
            })
            .await;
        })
        .await;
    })
}

This commit needs smol-rs/async-executor#4, so CI will fail.

[ghost]

What I like about the current behavior is that smol::run() is fully self-contained and does cleanup at the end of its invocation. That means no tasks will be "leaked" for the next invocation of smol::run() or until the thread exits.

Do you have a particular use case for the behavior proposed in this PR?

[oblique]

What I like about the current behavior is that smol::run() is fully self-contained and does cleanup at the end of its invocation. That means no tasks will be "leaked" for the next invocation of smol::run() or until the thread exits.

I didn't thought of that. I edited my commit to not use thread_local. Now I create LocalExecutor within closures.

Do you have a particular use case for the behavior proposed in this PR?

Yes, and it is very difficult to guarantee that Task::local is called on the main thread.

[ghost]

I think this is now a stale PR :) Thanks for the work on it, though!