Removed a totally unnecessary sort in AddAll

benchmark                  old ns/op    new ns/op    delta
BenchmarkUnion              93084064     62829349  -32.50%
Parallel intersection

With 8 cores, we can have a pretty good improvement.

benchmark                old ns/op    new ns/op    delta
BenchmarkIntersection     90831388     25528289  -71.89%

With 1 core there's a slight degradation.

benchmark                old ns/op    new ns/op    delta
BenchmarkIntersection     90831388    102541651  +12.89%