Why doesn't the `IntMap` and `IntSet` code use pointer equality checks to improve sharing and reduce allocations?

[sjakobi]

There's quite a bit of ptrEq usage in the Map and Set code. Why isn't there anything similar in the corresponding IntMap and IntSet code?

[sjakobi]

WIP patch for IntMap.difference in #1241.

If this works out, withoutKeys, restrictKeys, intersection, and delete could potentially be improved in a similar way.