Add impl<T> AsMut<T> for Exclusive<T>

[Jules-Bertholet]

cc #98407

@rustbot label T-libs-api

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[tgross35]

Nominated for libs-api to discuss the change #98407 (comment)

[tgross35]

Per #98407 (comment), this just needs to drop the existing get_mut.

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[Jules-Bertholet]

this just needs to drop the existing get_mut.

I would rather do this in a separate PR, as it is likely to be more contentious.

[bors]

☔ The latest upstream changes (presumably #147128) made this pull request unmergeable. Please resolve the merge conflicts.

[rustbot]

This PR was rebased onto a different master 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.