MultiDict and MultiSet should be either abstract traits or concrete implementations

[joshlemer]

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Currently the pattern between MultiDicts and MultiSets are different:

• MultDict is a concrete collection
• MultiSet is an abstract data type (trait) with a public concrete implementation MultiSetImpl.

I lean more towards the abstract data type approach of MultiSet, since it is consistent with the rest of the collections. Though the name MultiSetIml/BagImpl isn't very good. Perhaps CountedMultiSet/CountedBag extends MultiSet/Bag?

[julienrf]

If the intermediate trait is not needed or useful, I would remove it.