Rollup merge of #137928 - RalfJung:const_cell, r=m-ou-se

stabilize const_cell

``@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:
```rust
// 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 rust-lang/rust#131283

Author: matthiaskrgr