add PersistentMultimap and PersistentSet

[kfriedberger]

There are several cases in CPAchecker, where a PersistentSet or a PersistentMultimap (for a limited set of value-types) would be nice. Both can be based on PersistentMap, but implementing them directly in java-common-lib might be more efficient and less error-prone.

[kfriedberger]

There is at least a PersistentSet in CPAchecker that should be merged into java-common-lib.