Tracking issue for `entry_and_modify`

[mchlrhw]

Tracking issue for the and_modify method on hash map Entrys (added in #44734).

[mchlrhw]

The scope of this has changed to include the BTreeMap as well as HashMap on the advice of @kennytm.

[hedgehog1024]

Just wonder, is there any particular reason to keep this feature unstable? I personally find it pretty usable and convenient and it doesn't look like something that can break an unspoken invariant

[kennytm]

@hedgehog1024 The only reason these are kept unstable is because no one pushed for stabilization. Everyone is welcome to submit a PR to stabilize a library feature 😊.

1. Ensure the documentations, examples and implementations are correct. Ensure no missing edge cases. Ensure necessary tests are present.
2. Change all #[unstable] attributes related to that feature to #[stable].
3. Submit the PR.
4. Wait for team consensus.
5. Profit!

[hedgehog1024]

@kennytm Does such a PR need to remove the information about this feature from Unstable Book?

[kennytm]

@hedgehog1024 Yes. Those information should be moved into the documentation of the functions.

[mchlrhw]

@hedgehog1024 Thanks for your support of this feature, much appreciated 😄