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

[Coca162]

Submitting this to see if it can be done as a pr or if it needs to be a RFC.

Creates .flatten_ref() to get a immutable reference of a nested option and .flatten_mut() for mutable references. Return type is Option<&T> as &Option<T> would require returning a references to None which can't be done in constant functions.

ACP: rust-lang/libs-team#186

[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.

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[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.