Linear-time `PSet.plusAll` and `PSet.minusAll`

[jackfirth]

Skimming the source code, it seems the PSet implementations don't optimize plusAll and minusAll beyond looping over plus and minus. However, the Efficient sets - a balancing act paper that the IntTreePMap class links to includes a linear-time algorithm for set union and (via reference) intersection. Any chance that could be implemented? I have an application in a large distributed system where linear-time union and intersection could make a huge difference.