Rollup merge of #49727 - stjepang:cell-update, r=SimonSapin

Rollup merge of #49727 - stjepang:cell-update, r=SimonSapin

Add Cell::update

This commit adds a new method `Cell::update`, which applies a function to the value inside the cell.

Previously discussed in: https://github.com/rust-lang/rfcs/issues/2171

### Motivation

Updating `Cell`s is currently a bit verbose. Here are several real examples (taken from rustc and crossbeam):

```rust
self.print_fuel.set(self.print_fuel.get() + 1);

self.diverges.set(self.diverges.get() | Diverges::Always);

let guard_count = self.guard_count.get();
self.guard_count.set(guard_count.checked_add(1).unwrap());
if guard_count == 0 {
    // ...
}
```

With the addition of the new method `Cell::update`, this code can be simplified to:

```rust
self.print_fuel.update(|x| x + 1);

self.diverges.update(|x| x | Diverges::Always);

if self.guard_count.update(|x| x.checked_add(1).unwrap()) == 1 {
    // ...
}
```

### Unresolved questions

1. Should we return the old value instead of the new value (like in `fetch_add` and `fetch_update`)?
2. Should the return type simply be `()`?
3. Naming: `update` vs `modify` vs `mutate` etc.

cc @SimonSapin