chore: avoid cloning and use .map/.entry in Rust code

[kevinji]

There's no functional changes, just some code cleanup.

[codecov]

Codecov Report

Patch coverage: 95.94% and project coverage change: +16.60% 🎉

Comparison is base (7b1588b) 48.76% compared to head (1465773) 65.36%.

Additional details and impacted files

@@             Coverage Diff             @@
##           master     #181       +/-   ##
===========================================
+ Coverage   48.76%   65.36%   +16.60%     
===========================================
  Files          23        7       -16     
  Lines        9899     3560     -6339     
  Branches       86        0       -86     
===========================================
- Hits         4827     2327     -2500     
+ Misses       4715     1233     -3482     
+ Partials      357        0      -357     

Flag	Coverage Δ
go	?
rust	65.36% <95.94%> (+0.04%)	⬆️
typescript	?

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Files Changed	Coverage Δ
rust/src/compress.rs	96.87% <95.12%> (-0.05%)	⬇️
rust/src/api.rs	97.25% <95.45%> (+0.03%)	⬆️
rust/src/verify.rs	94.76% <100.00%> (-0.13%)	⬇️

... and 17 files with indirect coverage changes

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[romac]

Very nice, thanks!