A Persistent Java Collections Library
Implement sequenced collection interfaces
`OrderedPSet::contains` is O(N) as it inherits from `AbstractSet::contains`
`OrderedPMap::containsKey` is O(N) as it inherits from `AbstractMap::containsKey`
Linear-time `PSet.plusAll` and `PSet.minusAll`
Provide Stream collectors for persistent collections.
speed things up
optimize `PSet.intersect()`
avoid creating intermediate objects
fix deprecation warnings
shrink things down