Rollup merge of #70817 - yoshuawuyts:task-ready, r=dtolnay

Add core::task::ready! macro

This PR adds `ready!` as a top-level macro to `libcore` following the implementation of `futures_core::ready`, tracking issue https://github.com/rust-lang/rust/issues/70922. This macro is commonly used when implementing `Future`, `AsyncRead`, `AsyncWrite` and `Stream`. And being only 5 lines, it seems like a useful and straight forward addition to std.

## Example

```rust
use core::task::{Context, Poll};
use core::future::Future;
use core::pin::Pin;

async fn get_num() -> usize {
    42
}

pub fn do_poll(cx: &mut Context<'_>) -> Poll<()> {
    let mut f = get_num();
    let f = unsafe { Pin::new_unchecked(&mut f) };

    let num = ready!(f.poll(cx));
    // ... use num

    Poll::Ready(())
}
```

## Naming

In `async-std` we chose to nest the macro under the `task` module instead of having the macro at the top-level. This is a pattern that currently does not occur in std, mostly due to this not being possible prior to Rust 2018.

This PR proposes to add the `ready` macro as `core::ready`. But another option would be to introduce it as `core::task::ready` since it's really only useful when used in conjunction with `task::{Context, Poll}`.

## Implementation questions

I tried rendering the documentation locally but the macro didn't show up under `core`. I'm not sure if I quite got this right. I used the [`todo!` macro PR](https://github.com/rust-lang/rust/pull/56348/files) as a reference, and our approaches look similar.

## References

- [`futures::ready`](https://docs.rs/futures/0.3.4/futures/macro.ready.html)
- [`async_std::task::ready`](https://docs.rs/async-std/1.5.0/async_std/task/index.html)
- [`futures_core::ready`](https://docs.rs/futures-core/0.3.4/futures_core/macro.ready.html)

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