Implement stable-wakers RFC

[p-avital]

Implements the ABI-stabilize core::task::Waker RFC

[rustbot]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @m-ou-se (or someone else) soon.

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[rustbot]

Hey! It looks like you've submitted a new PR for the library teams!

If this PR contains changes to any rust-lang/rust public library APIs then please comment with @rustbot label +T-libs-api -T-libs to tag it appropriately. If this PR contains changes to any unstable APIs please edit the PR description to add a link to the relevant API Change Proposal or create one if you haven't already. If you're unsure where your change falls no worries, just leave it as is and the reviewer will take a look and make a decision to forward on if necessary.

Examples of T-libs-api changes:

• Stabilizing library features
• Introducing insta-stable changes such as new implementations of existing stable traits on existing stable types
• Introducing new or changing existing unstable library APIs (excluding permanently unstable features / features without a tracking issue)
• Changing public documentation in ways that create new stability guarantees
• Changing observable runtime behavior of library APIs

[workingjubilee]

Ironically, this PR is getting negative feedback from tidy due to the lack of stability attributes.

[p-avital]

Is writing an arbitrary feature name sufficient?

[workingjubilee]

Yes, you can just pick names out of a hat.

[bors]

☔ The latest upstream changes (presumably #110324) made this pull request unmergeable. Please resolve the merge conflicts.

[workingjubilee]

I believe the way the conversation in rust-lang/rfcs#3404 went suggests this will not be pursued further, please feel free to reopen and rebase (in that order) if I am wrong.