You're probably aware of this, but just in case you weren't.

Your union implementation overflows Int for:

iterate (\a -> union a a) a

I needed to implement something like your package for GHC and that's what I ended up with:

https://phabricator.haskell.org/diffusion/GHC/browse/master/compiler/utils/UniqDFM.hs;20c06143a7bfc6548351e610350a8e1c0d8a0bb9$128-136