Implement set union of many small arrays

[rgba]

Ops, I failed to realize at first that the ratio of uniques converges fast to 1/3, rather then decreasing indefinitely, what a fail!

@rgba

in the worst case scenario the size of the union would be orders of magnitude smaller than the size of a simple concatenation

Anyway, maybe we should move these last three post two post of us (me and @ivanpribec) to a new thread… Who to ask?