base64: improve perfs by using the base64-simd crate

[sylvestre]

No description provided.

[sylvestre]

Excellent results. We moved from slower to faster

 hyperfine --export-markdown base.md '/usr/bin/base64 /tmp/oneline_4G.txt' './target/release/coreutils base64 /tmp/oneline_4G.txt' './target/release/coreutils.prev base64 /tmp/oneline_4G.txt' -N --warmup 3
Benchmark 1: /usr/bin/base64 /tmp/oneline_4G.txt
  Time (mean ± σ):      4.901 s ±  0.054 s    [User: 3.904 s, System: 0.996 s]
  Range (min … max):    4.824 s …  4.971 s    10 runs

Benchmark 2: ./target/release/coreutils base64 /tmp/oneline_4G.txt
  Time (mean ± σ):      3.146 s ±  0.037 s    [User: 1.208 s, System: 1.937 s]
  Range (min … max):    3.096 s …  3.196 s    10 runs

Benchmark 3: ./target/release/coreutils.prev base64 /tmp/oneline_4G.txt
  Time (mean ± σ):      5.998 s ±  0.055 s    [User: 4.077 s, System: 1.918 s]
  Range (min … max):    5.882 s …  6.082 s    10 runs

Summary
  ./target/release/coreutils base64 /tmp/oneline_4G.txt ran
    1.56 ± 0.02 times faster than /usr/bin/base64 /tmp/oneline_4G.txt
    1.91 ± 0.03 times faster than ./target/release/coreutils.prev base64 /tmp/oneline_4G.txt

Command	Mean [s]	Min [s]	Max [s]	Relative
/usr/bin/base64 /tmp/oneline_4G.txt	4.901 ± 0.054	4.824	4.971	1.56 ± 0.02
./target/release/coreutils base64 /tmp/oneline_4G.txt	3.146 ± 0.037	3.096	3.196	1.00
./target/release/coreutils.prev base64 /tmp/oneline_4G.txt	5.998 ± 0.055	5.882	6.082	1.91 ± 0.03

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/timeout/timeout (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/misc/stdbuf (passes in this run but fails in the 'main' branch)

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/timeout/timeout (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/misc/stdbuf (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/misc/tee (passes in this run but fails in the 'main' branch)

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/timeout/timeout (fails in this run but passes in the 'main' branch)