union, diff and intersect could be better written

[rlgomes]

At the time I hacked these together just to have them exist but they know too much about the internals of flume and do all kinds of hackish things. I'd say if someone shows interest in flume that fixing these up to be better written without odd internals references to things like __meta__ so even if someone else wanted to write a set operator like one of these they could do so with ease.