Begin to implement PersistentHashSet. Introduces IPersistentSet.
Merge pull request #6 from patrickgombert/sets

Begin to implement PersistentHashSet. Introduces IPersistentSet.
Move logical and comparison into operators and add 'and' into operators
Merge pull request #7 from patrickgombert/logical-and

Move logical and comparison into operators and add 'and' into operators