Fix clippy issues in blocksort.rs

[folkertdev]

And some minor cleanup while we're at it.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Flag	Coverage Δ
fuzz-compress_then_decompress	50.81% <0.00%> (-0.33%)	⬇️
fuzz-decompress_chunked	31.02% <0.00%> (-1.40%)	⬇️
test-aarch64-apple-darwin	?
test-aarch64-unknown-linux-gnu	89.37% <100.00%> (-0.03%)	⬇️
test-i686-unknown-linux-gnu	?
test-x86_64-apple-darwin	89.45% <100.00%> (-0.02%)	⬇️
test-x86_64-unknown-linux-gnu	89.45% <100.00%> (-0.02%)	⬇️

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

Files with missing lines	Coverage Δ
bzip2recover.rs	94.77% <100.00%> (-0.02%)	⬇️
libbz2-rs-sys/src/blocksort.rs	99.12% <100.00%> (-0.02%)	⬇️

... and 1 file with indirect coverage changes

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[folkertdev]

Looks like it's a nice performance improvement too actually

Benchmark 1 (402 runs): target/release/examples/compress-baseline rs 1 tests/input/quick/sample3.ref
  measurement          mean ± σ            min … max           outliers         delta
  wall_time          12.4ms ±  201us    11.9ms … 14.3ms         16 ( 4%)        0%
  peak_rss           3.27MB ± 23.2KB    3.15MB … 3.28MB         13 ( 3%)        0%
  cpu_cycles         49.5M  ±  649K     48.5M  … 58.3M          13 ( 3%)        0%
  instructions        218M  ±  269       218M  …  218M           1 ( 0%)        0%
  cache_references   4.69M  ±  126K     4.08M  … 5.00M           6 ( 1%)        0%
  cache_misses        273K  ± 35.3K     90.6K  …  368K          19 ( 5%)        0%
  branch_misses      82.2K  ±  278      81.6K  … 84.4K          11 ( 3%)        0%
Benchmark 2 (416 runs): target/release/examples/compress rs 1 tests/input/quick/sample3.ref
  measurement          mean ± σ            min … max           outliers         delta
  wall_time          12.0ms ±  140us    11.6ms … 13.2ms         19 ( 5%)        ⚡-  3.1% ±  0.2%
  peak_rss           3.27MB ± 16.9KB    3.15MB … 3.28MB          7 ( 2%)          +  0.1% ±  0.1%
  cpu_cycles         47.7M  ±  499K     46.8M  … 53.4M          15 ( 4%)        ⚡-  3.6% ±  0.2%
  instructions        218M  ±  253       218M  …  218M           1 ( 0%)          +  0.2% ±  0.0%
  cache_references   4.70M  ±  109K     4.40M  … 5.01M           5 ( 1%)          +  0.2% ±  0.3%
  cache_misses        259K  ± 33.0K      155K  …  336K          10 ( 2%)        ⚡-  5.1% ±  1.7%
  branch_misses      82.4K  ±  318      81.6K  … 83.6K          12 ( 3%)          +  0.2% ±  0.0%