[associative] Feature requests: HashSet implementation and document asymptotic complexity, ordering

[fredkilbourn]

Feature requests, subject says it all for the most part:

1. Would love a HashSet implementation, seems like the most obvious missing collection here.
2. Would be nice to see more detailed documentation of expected time complexity for the different collection types for insert/lookup/iteration/etc. After looking at the code, saw that some of the structures seem to be O(N) for insert/lookup which isn't great and would be worth noting.
3. Would be nice to have more detailed documentation about whether collections preserve insert order on iteration.

Nice library overall though, thanks to the maintainers for their time on the project.