Suggestion: add 'multiset' and 'bag' tags to repository, README and Cargo.toml

[mjpieters]

A counter is a multiset, AKA a bag. The latter term is very generic, but still something people search for when they look for the datatype. However, the counter project makes no mention of these terms, in the README, the GitHub tags or the Cargo.toml keywords list.

There are currently no Rust projects with the multiset tag on GitHub, and the multiset projects on Crates.io I looked at are not nearly as flexible and versatile as counter is. E.g. the hashbag library doesn't offer intersections, unions, subset or superset functionality nor does it offer syntactic sugar support the way counter does.

However, anyone looking for a multiset will find those other projects and not this one.