add assertion to check that UniqFM is only passed "positive" uniques
The insertion code in UniqFM fails if a unique key produces a negative FastInt. I've added an assertion to check that each insertion uses a positive Unique. Where do the negative uniques come from? Both Simom M and I have run into this problem when computing hashes for data structures. In both cases, we have avoided the problem by ensuring that the hashes remain positive.