Rollup merge of #119984 - kpreid:waker-noop, r=dtolnay

Change return type of unstable `Waker::noop()` from `Waker` to `&Waker`.

The advantage of this is that it does not need to be assigned to a variable to be used in a `Context` creation, which is the most common thing to want to do with a noop waker. It also avoids unnecessarily executing the dynamically dispatched drop function when the noop waker is dropped.

If an owned noop waker is desired, it can be created by cloning, but the reverse is harder to do since it requires declaring a constant. Alternatively, both versions could be provided, like `futures::task::noop_waker()` and `futures::task::noop_waker_ref()`, but that seems to me to be API clutter for a very small benefit, whereas having the `&'static` reference available is a large reduction in boilerplate.

[Previous discussion on the tracking issue starting here](https://github.com/rust-lang/rust/issues/98286#issuecomment-1862159766)

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