MultiSets

Thanks for this really cool lib. As far as I see, it does not contain any MultiSet implementation (i.e. each object in a set has a number of times it appears in the set… so a bit like a non-ordered list). In particular I'd love a way to check if a multi-set is a subset of another multiset (i.e. the number of items is smaller for all items). I can certainly simulate it via a bunch of `groupBy`, and `every`, but it is possibly less efficient since we need to recompute everything every time we change an object.