fix(index): additional bucket entry verification with checksum

[zkat]

Cacache was just using length for these entries before. As I thought more about consistency checks and verification, I wondered if it was worth the cost to just straight-up hashing that entry string. Turns out the cost was minimal, and I think this brings entry verification into the "tremendous confidence in consistency" realm, which feels like a warm-fuzzy guarantee to be able to make.

Benchmarks

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     index.find cache hit
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  10610 ops/s @ ~0.094ms/op (+1.72% ±0.99%)
  Sampled 83 in 5.93s.
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     index.find cache miss
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  18257 ops/s @ ~0.055ms/op (-3.42% ±1.58%)
  Sampled 83 in 6.01s.
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     index.insert() different files
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  6464 ops/s @ ~0.155ms/op (+4.21% ±1.05%)
  Sampled 84 in 5.98s.
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     index.insert() same file
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  6461 ops/s @ ~0.155ms/op (+6.11% ±0.85%)
  Sampled 83 in 6.04s.
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