Better multiset serialization / deserialization

(moved from https://github.com/FasterXML/jackson-datatype-guava/issues/51 by @devinrsmith)

Currently, multisets' serialization size is proportional to Multiset.size() as opposed to Multiset.entrySet().size().

For example, if I have do Multiset.setCount("my_key", 1000), "my_key" is repeated 1000 times in the output. It would be much more appropriate if it behaved liked Map.

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I'm not familiar w/ the intricacies of jackson serialization / deserialization. I'd be pretty surprised if there is widespread use of serializing Multisets (using the existing jackson library)... I would have guessed a lot of other people would be bringing up this exact issue if they were using it. Does jackson ever create hard-breaks in backwards compatibility?