An unordered multiset/bag implementation backed by `HashMap`.

A bag, unlike a set, allows duplicate values, and keeps track of how many duplicates each value holds. This type of collection is often referred to as an unordered multiset (see also C++'s `std::unordered_multiset`).

## License

Licensed under either of

- Apache License, Version 2.0 (LICENSE-APACHE or http://www.apache.org/licenses/LICENSE-2.0)
- MIT license (LICENSE-MIT or http://opensource.org/licenses/MIT)

at your option.

## Contribution

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.