make Cell unstably const
Now that we can do interior mutability in `const`, most of the Cell API can be `const fn`. :) The main exception is `set`, because it drops the old value. So from const context one has to use `replace`, which delegates the responsibility for dropping to the caller.
Tracking issue: https://github.com/rust-lang/rust/issues/131283
`as_array_of_cells` is itself still unstable to I added the const-ness to the feature gate for that function and not to `const_cell`, Cc #88248.
r? libs-api
388c10b2ac