I talked to Chris and had a couple of points: 1) this method is necessary for implementing WrapperMap/toMap(), so toMap().values isn't an option without it. 2) values should arguably de-dupe (by identity), especially if you look at Multimap as a M-N mapping. If values did de-dupe, then this could be called `allValues`. Chris points out that de-duping makes sense especially in a SetMultimap where the dupes could be quite confusing.

But since _WrapperMap takes a `_BaseMultimap` and not just a `Multimap`, I can make this getter private and defer the decision.