Caching

[langston-barrett]

This paper points out that sundry situations can cause test-case reducers to retry inputs they've already attempted. It's not clear to me if treereduce does this; in either case I should introduce a cache mapping test-cases (or maybe their hashes) to the result of the interestingness test. At first, treereduce should produce an error when it's about to test an already-tested variant so that I can uncover why it's happening and perhaps just avoid it in the first place. If that's not possible, then the cache should be maintained as it will reduce testing times!

https://ieeexplore.ieee.org/abstract/document/7962327