get_many_mut with variable number of keys

get_many_mut on slices (https://github.com/rust-lang/rust/pull/83608) does not need to handle a variable number of keys (although it might be nice) because, worse case scenario, you can sort your input array of indexes and iterate over it with successive split_at_mut.

In this case however, that's not an option, as HashMaps don't have the equivalent of split_at_mut.
So it may be useful to be able to lookup a variable (somewhat large) number of distinct keys at the same time in a HashMap, and get mutable references back.

That is in particular needed when we have a storage of many entries, and we want to run a round of updates on distinct subsets of keys, and want to run these in parallel.

Having get_many_mut follow a similar model to what is described at https://github.com/rust-lang/rust/pull/83608#pullrequestreview-966536956 and providing an interface that allows for this would consequently be useful - in fact, that is something I currently would use.

Start with the get_many_mut interface discussed in Rust PR 83608 and compare its slice-based model with HashMap access in hashbrown. Define how a variable, somewhat large number of distinct keys should yield mutable references and support parallel updates; the issue does not name a file or test to run.