Map-like data structure #20
Description
This data structure should use the same database schema as PersistentCollection, but calls to #get(SomeKey) should be constant time operations. Further, composite keys need to be kept in mind. I'm proposing that some key extractor function is provided to the map on creation, which will be used to extract a key from a UniqueData. For the time being, this only needs to be implemented for one to many relations.