update extension strategy to match BLAST

[thatbudakguy]

the actual implementation of BLAST extends in both directions, which would help us because it would allow discarding matches immediately after the seeding step (instead of needing to extend and align prior to discarding in order to make sure the match meets the criteria for being discarded).

BLAST also uses a poisson method to do what we called "condensing" high-similarity regions; BLAST2 used the simpler sum-of-scores method. In either case it may be worth revisiting this step to see if it affords performance gains.

see Vesanto 2019 for a broad overview of using BLAST for text reuse.

[thatbudakguy]

#151 partially implements this (the extend-in-both directions part), but there's a lot more we could do. in particular, using BLAST's smart method of preselecting high-scoring seeds would probably help us a lot.