A map that creates a random handle on insertion to use when retrieving.
The raison d'ĂȘtre for this map is:
- You want to put something in a map, but you have no key. Means you do
not want to use a [`HashMap`
](https://doc.rust-lang.org/std/collections/struct.HashMap.html) or
[`BTreeMap`
](https://doc.rust-lang.org/std/collections/struct.BTreeMap.html).
- You want to forget the details of what you put in to later retrieve it
with a simple handle, and you are not interested in how many equal
objects you insert. Means you do not want to use a [`HashSet`
](https://doc.rust-lang.org/std/collections/struct.HashSet.html) or
[`HashMultiSet`
](https://docs.rs/multiset/latest/multiset/struct.HashMultiSet.html).
- You want a persistent handle to refer to the item you put in the map.
Means you do not want to use a `Vec`.
The implementation uses a `HashMap` that does not actually hash. The
contained `HashMap` can be borrowed (`.as_hash_map()`), so all `HashMap`
functions that do not change the map are at your disposal.