Reimplement Set collection

[mappum]

We got rid of it during #36

[mappum]

Since keys are limited in length to 256 bytes, the pattern of storing values as keys so they are indexed seems prone to hitting this limit. We could probably approach this by implementing a Set type which uses hashes of the values as store keys, and stores the encoded value in the store value. This is maybe easier to implement on top of EntryMap since it has a similar API but I'm not sure if it all maps 1:1.

Since the ordering would be seemingly random (but still deterministic), it may be a good choice to not even support iteration in order to disincentivize writing code which iterates over too many values.