Tidy up maybe_uninit_as_bytes API

[SUPERCILEX]

• Follow mut naming conversion
• Move methods to extensions

The diff is super weird because it misunderstood what was moved.

[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

[rust-highfive]

r? @joshtriplett

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[scottmcm]

Thanks for the PR!

While I'm sometimes willing to add new APIs without libs-api assent, I'm much more worried about churning signatures without contacting them to see if they like them, since it would be a shame to find out they didn't and have to revert it and have impacted nightly users unnecessarily.

Could you please file an ACP for this set of changes (I think #103128 #103130 #103131 could probably all be one) to get a "yup, that seems better" from the team?

@rustbot author

[SUPERCILEX]

Sounds good! Created rust-lang/libs-team#123

[SUPERCILEX]

Gonna nuke these methods, see rust-lang/libs-team#122 (comment)