Generalized merge API for sets

[sjakobi]

The idea to add a generalized merge API in the style of the Data.[Int]Map.Merge.* modules, but for Set and IntSet, has most recently come up in the context of the symmetric difference proposal: https://mail.haskell.org/pipermail/libraries/2020-June/030633.html

[meooow25]

#1169 adds merge for Set.

Defining an API for IntSet is more complicated due to the packed tips. For instance, if one wants to define intersection via merge and wants it to be around as efficient as the existing intersection, the one-element-at-a-time approach of IntMap/Map/Set is poor compared to being able to do bm' = bm1 .&. bm2.

I'll try to think if there a decent way to offer this.