[MRG] Fix new clippy warnings from Rust 1.49

[luizirber]

CI started failing after Rust 1.49 was released, since 5 new clippy warnings showed up. This PR fixes them.

Checklist

• Is it mergeable?
• make test Did it pass the tests?
• make coverage Is the new code covered?
• Did it change the command-line interface? Only additions are allowed
  without a major version increment. Changing file formats also requires a
  major version number increment.
• Was a spellchecker run on the source code and documentation after
  changes were made?

[codecov]

Codecov Report

Merging #1267 (d22470e) into latest (ca201cf) will decrease coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           latest    #1267      +/-   ##
==========================================
- Coverage   88.71%   88.67%   -0.04%     
==========================================
  Files         125      125              
  Lines       18238    18231       -7     
  Branches     1434     1434              
==========================================
- Hits        16180    16167      -13     
- Misses       1812     1818       +6     
  Partials      246      246              

Flag	Coverage Δ
python	93.95% <ø> (ø)
rust	67.33% <100.00%> (-0.23%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/core/src/index/sbt/mod.rs	67.07% <100.00%> (-0.31%)	⬇️
src/core/src/sketch/hyperloglog/mod.rs	81.76% <100.00%> (+0.55%)	⬆️
src/core/src/encodings.rs	77.77% <0.00%> (-1.59%)	⬇️
src/core/src/sketch/minhash.rs	91.31% <0.00%> (-1.06%)	⬇️
src/core/src/signature.rs	59.39% <0.00%> (-0.73%)	⬇️
src/core/src/ffi/minhash.rs	0.00% <0.00%> (ø)
src/core/src/ffi/nodegraph.rs	0.00% <0.00%> (ø)
src/core/src/ffi/cmd/compute.rs	0.00% <0.00%> (ø)
src/core/tests/minhash.rs	99.76% <0.00%> (+0.23%)	⬆️

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[luizirber]

Ready for review and merge @ctb