Add gb_tree based OR-Set.
Add gb_tree based OR-Set.

Add a gb_tree based OR-Set that can perform a merge without requring the
entire data structure be loaded into memory.

lasp-lang/lasp,ergl/lasp,ergl/lasp,lasp-lang/lasp,ergl/lasp,lasp-lang/lasp,ergl/lasp,lasp-lang/lasp