Create Atomic<T> type alias (rebase)

[GrigorenkoPV]

Rebase of #130543.

Additional changes:

• Switch from allow to expect for private_bounds on AtomicPrimitive
• Unhide AtomicPrimitive::AtomicInner from docs, because rustdoc shows the definition pub type Atomic<T> = <T as AtomicPrimitive>::AtomicInner; and generated links for it.
  • NonZero did not have this issue, because they kept the new alias private before the direction was changed.
• Use Atomic<_> in more places, including inside Once's Futex. This is possible thanks to declare_interior_mutable_const, borrow_interior_mutable_const: resolve <T as Trait>::AssocT projections rust-clippy#14125

The rest will either get moved back to #130543 or #130543 will be closed in favor of this instead.

[rustbot]

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[rustbot]

Failed to set assignee to ghost: invalid assignee

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[ChrisDenton]

I've removed myself from the assigned reviewer for now but feel free to assign to me again (or r? libs) when you're ready.

[bors]

☔ The latest upstream changes (presumably #136572) made this pull request unmergeable. Please resolve the merge conflicts.

[GrigorenkoPV]

@rustbot ready
r? libs

[bors]

☔ The latest upstream changes (presumably #138279) made this pull request unmergeable. Please resolve the merge conflicts.