Rollup merge of #94647 - Urgau:hash-map-many-mut, r=Amanieu

Rollup merge of #94647 - Urgau:hash-map-many-mut, r=Amanieu

Expose `get_many_mut` and `get_many_unchecked_mut` to HashMap

This pull-request expose the function [`get_many_mut`](https://docs.rs/hashbrown/0.12.0/hashbrown/struct.HashMap.html#method.get_many_mut) and [`get_many_unchecked_mut`](https://docs.rs/hashbrown/0.12.0/hashbrown/struct.HashMap.html#method.get_many_unchecked_mut) from `hashbrown` to the standard library `HashMap` type. They obviously keep the same API and are added under the (new) `map_many_mut` feature.

- `get_many_mut`: Attempts to get mutable references to `N` values in the map at once.
- `get_many_unchecked_mut`: Attempts to get mutable references to `N` values in the map at once, without validating that the values are unique.