Add `SetOrderedMultimap` type

[sgodwincs]

Same as ListOrderedMultimap but only a unique set of values can be associated with a given key. This will require Eq and Hash bounds on the Value type.

[sgodwincs]

Working on this now. I believe there's a fairly "simple" implementation to be had by reusing ListOrderedMultimap and keeping a separate values: HashSet<Index<...>> to reference back into the internal values list.

[sgodwincs]

OK, I may have underestimated the code complexity here, reusing ListOrderedMultimap directly is infeasible without just opening all of its internals. Going forward with just reimplementing a lot of it in SetOrderedMultimap.

Good news is that the approach looks to be working under the feature and performance constraints I want.