Fix clippy lints

[Rawk]

Fix a lot of Clippy lints. Mostly for files under programs/, but also enables unused_assignments for lib/.

[codecov]

Codecov Report

❌ Patch coverage is 14.48087% with 939 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
programs/zstdcli.rs	0.00%	306 Missing ⚠️
programs/fileio.rs	0.00%	209 Missing ⚠️
programs/util.rs	0.00%	95 Missing ⚠️
lib/compress/zstd_compress.rs	19.79%	77 Missing ⚠️
programs/benchzstd.rs	0.00%	38 Missing ⚠️
programs/fileio_asyncio.rs	0.00%	38 Missing ⚠️
lib/legacy/zstd_v06.rs	44.89%	27 Missing ⚠️
lib/compress/zstd_fast.rs	0.00%	25 Missing ⚠️
lib/compress/zstd_opt.rs	0.00%	19 Missing ⚠️
programs/dibio.rs	0.00%	19 Missing ⚠️
... and 18 more

Flag	Coverage Δ
test-aarch64-apple-darwin	32.46% <14.29%> (-0.70%)	⬇️
test-aarch64-unknown-linux-gnu	31.72% <14.20%> (+0.06%)	⬆️
test-i686-unknown-linux-gnu	31.78% <14.20%> (+0.05%)	⬆️
test-x86_64-unknown-linux-gnu	32.74% <14.38%> (-0.21%)	⬇️

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

Files with missing lines	Coverage Δ
lib/common/entropy_common.rs	94.24% <100.00%> (-0.41%)	⬇️
lib/common/fse_decompress.rs	94.52% <100.00%> (-0.48%)	⬇️
lib/common/pool.rs	58.25% <100.00%> (-0.21%)	⬇️
lib/compress/hist.rs	74.18% <ø> (-0.21%)	⬇️
lib/compress/zstd_double_fast.rs	77.15% <100.00%> (-0.33%)	⬇️
lib/compress/zstd_preSplit.rs	71.26% <100.00%> (-0.17%)	⬇️
lib/decompress/zstd_decompress_block.rs	76.24% <ø> (-0.02%)	⬇️
lib/dictBuilder/divsufsort.rs	90.96% <100.00%> (-0.35%)	⬇️
lib/dictBuilder/fastcover.rs	78.67% <100.00%> (+2.44%)	⬆️
lib/dictBuilder/zdict.rs	79.26% <100.00%> (-2.16%)	⬇️
... and 29 more

... and 2 files with indirect coverage changes

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