Use digest instead of hexdigest when caching reduction attempts.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 83.25%. Comparing base (95341da) to head (9d37b07).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #94 +/- ## ========================================== - Coverage 83.40% 83.25% -0.15% ========================================== Files 13 13 Lines 1476 1463 -13 ========================================== - Hits 1231 1218 -13 Misses 245 245 ```

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