[breaking change] Revert Entry behaviour to take keys by value.

[dylanede]

This reverts the behaviour of the Entry API to take keys by value.

It fixes #20625.

This is not the only way of fixing the issue, but this solution is available in the case that a by-value API becomes the preferred choice.

[breaking-change]

[rust-highfive]

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

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. The way Github handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

Please see CONTRIBUTING.md for more information.

[alexcrichton]

Closing in favor of #20653, thanks!