Implement flatten for Option<&Option<T>> and Option<&mut Option<T>>

[Coca162]

Adds .flatten_ref() to get a immutable reference of a nested option and .flatten_mut() for mutable references.

ACP: rust-lang/libs-team#186
Tracking Issue: #149221

[rustbot]

r? @cuviper

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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

[cuviper]

Please do create an API Change Proposal as linked above.

@rustbot label +T-libs-api -T-libs
r? libs-api

[joshtriplett]

r? libs-api

[bors]

☔ The latest upstream changes (presumably #130829) made this pull request unmergeable. Please resolve the merge conflicts.

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.