Fact index

[alex-dixon]

• Remove rules that find and retract facts based upon ancestor hierarchy
• Reduce schema definitions to facts that belong to the persistence layer only
• Make default hierarchy for all facts #{:all :one-to-one}
• Store fn created by util/make-ancestors-fn in state/ancestors-fn
• Collapse Datomic's uniqueness distinctions into :unique and continue to perform upserts on unique attributes following research in #32
• Add state/fact-index atom with e-a index for :one-to-one facts, a v index for :unique. E.g.:

  :one-to-one -> e -> a -> fact
  :unique -> a -> v -> fact

• Support :one-to-many but do not track in fact index (doesn't seem necessary)
• Add fact-index-path fn that uses state/ancestor-fn to determine a fact's location in the fact index according to its type
• Add find-in-fact-index: If existing fact in index, replaces it and returns existing fact so it can be retracted. Else writes new fact to index and returns nil (no retraction)
• All insert fns index the facts they receive as arguments and receive a list of facts to be retracted. Both are diffed using clojure.set/difference to avoid performance degradation when retracting many facts using Clara's retract.
• All retract fns remove the facts they receive as arguments from fact-index
• Added tests for new fns
• Modified existing tests that were not concerned with adhering to :one-to-one or other schema distinctions to work as previously with added changes to insert/retract