Use a more sophisticated representation of datoms to insert in order to detect errors

[rnewman]

#531 is the original issue. #533 made it so that an error is thrown, but it's not a useful error: it's very internal. It would be better to detect the two datoms that failed.

One way to do this is to build a trie of e -> a -> v instead of a vector of ReducedEntity, which not only eliminates duplication and perhaps gives us a more efficient time with insertion, but also lets us exactly detect duplicates. If we can build this trie as part of the last transact generation pass, then this might be cheap or even a saving.