Accidentally-Quadratic HashMaps

TL;DR: Unless you’re careful, using foldMap to build HashMaps (from unordered-containers) can take O(n²) time. This can be avoided by:

1. Building a Map (from containers) instead;

2. Using foldr or foldl with insert; or

3. Using foldMap to build an intermediate list [(k, v)] and then building the HashMap with fromList.

Shout-out to Luke Worth who found this out while we were chasing down performance problems.