tokio: Stabilize LocalRuntime

[Daksh14]

Motivation

After LocalRuntime was implemented as per the issue #6739 I think its time to stabilize it so we don't need to set the RUSTFLAGS when using it

Solution

Remove cfg unstable in docs and places where it was initially implemented, I have tested the macro

#[tokio::main(flavor = "local")]
async fn main() {
    println!("hello");
}

And using it just like

use tokio::runtime::LocalRuntime;

fn main() {
    let rt = LocalRuntime::new().unwrap();

    rt.block_on(async {
        println!("{:?}", "hello");
    });

    println!("hello");
}

[ADD-SP]

I think the LocalRuntime is useful. However, I'd like to hit the brakes for now, there are more works we need to do before stabilizing it in the code. Removing the tokio_unstable from the code should be the last work for stabilization.

[ADD-SP]

1. Shall we deprecate the tokio::task::LocalSet and tokio_util::task::LocalPoolHandle?
2. There are many overlaps between the LocalRuntime and CurrentThreadRuntime, what's the future of the current thread runtime? Any guideline to downstream?
3. Shall we add the tokio::spawn_local? What's the behavior when it is called from current_thread or multi_thread runtime?
4. Any other impacts to the existing usage of tokio, tokio-stream, and tokio-util?

[Darksonn]

Relevant to this: #6741

[takeoverjp]

LocalRuntime is very helpful for me, thanks!

I think it's better to add description of flavor = "local in the doc comment of tokio::test as well as tokio::main.
Like the following:
(Sorry, but I couldn't comment it on source code due to GitHub UI)

/// # local runtime
///
/// The default test runtime is the [local runtime]. The [local runtime] is similar to the current-thread
/// but supports[`task::spawn_local`](../tokio/task/fn.spawn_local.html).
///
/// ```no_run
/// #[tokio::test(flavor = "local")]
/// async fn my_test() {
///     assert!(true);
/// }
/// ```