Optimize Comparators

[tylerwilding-wk]

We should take some time to revisit comparator optimization. In particular, for most index-based operations they are the dominant factor by a fair margin. I trust Timothy Dean's work so far on them has them near-optimal for being written in clojure, but, given the importance that they be fast, I think we should consider taking the time to rewrite them explicitly in java on top of an immutable PackedDatom class. This would let us strip out the overhead of some of the inherently polymorphic clojure core functions, like nth: https://github.com/clojure/clojure/blob/master/src/jvm/clojure/lang/RT.java#L866