[EXP] try out making a cover for the prefetch results

[ctb]

Trying out some optimization stuff per #1852

[codecov]

Codecov Report

Merging #1853 (b3cafbf) into latest (27826d8) will increase coverage by 8.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           latest    #1853      +/-   ##
==========================================
+ Coverage   82.25%   90.28%   +8.02%     
==========================================
  Files         119       89      -30     
  Lines       12918     8738    -4180     
  Branches     1727     1729       +2     
==========================================
- Hits        10626     7889    -2737     
+ Misses       2026      581    -1445     
- Partials      266      268       +2     

Flag	Coverage Δ
python	90.28% <100.00%> (-0.04%)	⬇️
rust	?

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Impacted Files	Coverage Δ
src/sourmash/index/__init__.py	96.16% <100.00%> (-0.30%)	⬇️
src/sourmash/commands.py	88.07% <0.00%> (-0.25%)	⬇️
src/core/src/sketch/hyperloglog/estimators.rs
src/core/src/index/revindex.rs
src/core/src/encodings.rs
src/core/src/signature.rs
src/core/src/ffi/signature.rs
src/core/src/from.rs
src/core/src/index/mod.rs
... and 23 more

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