base(nc|32|64): Optimize performances reduction memset

[mattsu2020]

Summary

• Allocate read buffers with capacity and reuse their spare capacity to avoid zeroing overhead in fast encode/decode paths.
• Switch fast encode/decode to read into spare capacity via slice::from_raw_parts_mut, keeping safety comments to document the initialization guarantees.
• Tidy base_common.rs imports for consistency.

related https://github.com/uutils/coreutils/issues/9621

[codspeed-hq[bot]]

CodSpeed Performance Report

Merging #9632 will improve performances by 3.15%

_{Comparing mattsu2020:base32_performance_check (27b72b4) with main (2000af8)}

Summary

⚡ 2 improvements
✅ 125 untouched
⏩ 6 skipped[^skipped]

Benchmarks breakdown

	Benchmark	BASE	HEAD	Change
⚡	b64_decode_synthetic	169.8 µs	164.6 µs	+3.15%
⚡	b64_decode_ignore_garbage_synthetic	169.7 µs	164.6 µs	+3.14%
[^skipped]: 6 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports.

[github-actions[bot]]

GNU testsuite comparison:

Skipping an intermittent issue tests/tail/overlay-headers (passes in this run but fails in the 'main' branch)

[github-actions[bot]]

GNU testsuite comparison:

Skipping an intermittent issue tests/tail/overlay-headers (passes in this run but fails in the 'main' branch)

[sylvestre]

i am not a fan of adding unsafe. would it be possible to find another way? thanks

[github-actions[bot]]

GNU testsuite comparison:

Congrats! The gnu test tests/cksum/cksum is no longer failing!

[sylvestre]

is it ready for review?

[github-actions[bot]]

GNU testsuite comparison:

Skipping an intermittent issue tests/tail/overlay-headers (passes in this run but fails in the 'main' branch)