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`]).

  [`std::unordered_multiset`]: http://www.cplusplus.com/reference/unordered_set/unordered_multiset/

## License

Licensed under either of

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