stabilize const_cell

[RalfJung]

@rust-lang/libs-api @rust-lang/wg-const-eval I see no reason to wait any longer, so I propose we stabilize the use of Cell in const fn -- specifically the APIs listed here:

// core::cell

impl<T> Cell<T> {
    pub const fn replace(&self, val: T) -> T;
}

impl<T: Copy> Cell<T> {
    pub const fn get(&self) -> T;
}

impl<T: ?Sized> Cell<T> {
    pub const fn get_mut(&mut self) -> &mut T;
    pub const fn from_mut(t: &mut T) -> &Cell<T>;
}

impl<T> Cell<[T]> {
    pub const fn as_slice_of_cells(&self) -> &[Cell<T>];
}

Unfortunately, set cannot be made const fn yet as it drops the old contents.

Fixes #131283

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[dtolnay]

@rfcbot fcp merge

[rfcbot]

Team member @dtolnay has proposed to merge this. The next step is review by the rest of the tagged team members:

• @Amanieu
• @BurntSushi
• @dtolnay
• @joshtriplett
• @m-ou-se

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

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[RalfJung]

@Amanieu @BurntSushi @joshtriplett friendly FCP checkbox reminder :)

[rfcbot]

🔔 This is now entering its final comment period, as per the review above. 🔔