Add `Option::transpose_fn` methods

[nvzqz]

Converts an Option of an Fn into an Fn that returns an Option.

ACP: https://github.com/rust-lang/libs-team/issues/166 [Accepted]

[rustbot]

r? @scottmcm

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[scottmcm]

Thanks for the PR! You'll need to file an ACP to get a libs-api second before we add a new unstable API, as described in https://std-dev-guide.rust-lang.org/feature-lifecycle/api-change-proposals.html.

[rustbot]

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[nvzqz]

@scottmcm thanks for the heads up! I submitted https://github.com/rust-lang/libs-team/issues/166

[bors]

:umbrella: The latest upstream changes (presumably #110393) made this pull request unmergeable. Please resolve the merge conflicts.

[Dylan-DPC]

@nvzqz this acp has been merged so if you can resolve the conflicts we can mark this as ready for review

[nvzqz]

The ACP wasn't approved because:

...of the many ways in which this transformation can be expressed, transpose_fn is the least readable. For example, simple doing |arg| Some(f?(arg)) is much clearer.

I don't feel strongly for or against my proposal so I don't intend to go against the libs API team's opinion.