This implementation of fingerprint indexing should take types into account, and also only traverse the relevant parts of the tree.
Right now it's not more efficient than Fingerprint (see the benchmarks in ../tests/run_bench.ml)
TODO: pre-index on types?
val fp3d : fingerprint_funval fp3w : fingerprint_funval fp4d : fingerprint_funval fp4m : fingerprint_funval fp4w : fingerprint_funval fp5m : fingerprint_funval fp6m : fingerprint_funval fp7 : fingerprint_funval fp7m : fingerprint_funval fp16 : fingerprint_funmodule Make (X : Set.OrderedType) : sig ... end