add missing {forward,backward}_overflowing impls

[tpdenk]

Cause: rust-lang/rust#155114

One problem: Kani bundles nightly-2025-11-21 (their main bundles nightly-2025-12-31 as of 2026-07-09 (yesterday at the time of writing) as per model-checking/kani#4624), meaning I can't add Kani proofs. Once they bundle nightly-2026-07-10 or later, you can do this: https://github.com/rust-osdev/x86_64/compare/4bc97dadf4481e6d8ce858c0ca6413e351322c12..9bf986f2282f691ccc856087bf7a86cd41316c03 (without disabling Kani of course).

Fixes #594

[zhangxuan2011]

ref #594

Haha, Just find this problem several hrs ago, and a PR was came out quickly :))) (my eng not good)

[tpdenk]

Today is the first day this problem even exists from what I can tell.
If you have a project that relies on nightly and breakages like these are really critical for you, you should really pin your nightly version (I've updated #594 (comment))

[zhangxuan2011]

Today is the first day this problem even exists from what I can tell. If you have a project that relies on nightly and breakages like these are really critical for you, you should really pin your nightly version (I've updated #594 (comment))

I did, but this is the temporary fix for now.

[tpdenk]

https://github.com/tpdenk/x86_64/actions/runs/29087392922/job/86344256732

Seems like semver checks uses rust stable? Idk why it does, but that's obv going to fail.

[zhangxuan2011]

https://github.com/tpdenk/x86_64/actions/runs/29087392922/job/86344256732

Seems like semver checks uses rust stable? Idk why it does, but that's obv going to fail.

idk the author will update it or not...

[Freax13]

Thank you for your contribution!

I had one small question, once resolved, this PR should be ready to be merged.

[Freax13]

LGTM, thanks!

[zhangxuan2011]

LGTM, thanks!

So i hope the new version of this crate will be published as soon as possible😄👏