Fix edge cases

A number of edge cases and off-by-one conditions have been fixed. To try to achieve some diversity in sieving results, FACTOR_FINDER works inward by batch from both ends of the sieving interval, like PRIME_PROVER mode. Also, pardon the breaking change (which should probably be accounted semantically), but PRIME_PROVER is the better name for that method, than PRIME_SOLVER.

Full Changelog: v6.3.1...v6.4.0

sha1sum results:
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