whoami: bypass clap for performance

[oech3]

Summary
  whoami-bypass ran
    1.05 ± 0.24 times faster than whoami

[sylvestre]

not sure it is worse the added complexity:

• the perf diff isn't that important
• i don't think it is a perf critical command

[oech3]

FILE, --help, and --version only commands are not changed too many times.
So I think this is not too pain to maintain.

An another solution is do that at #4254 instead.

[sylvestre]

another solution is to let it this way ;)
i don't think anyone cares about whoami performances

[oech3]

How about adding fast path to uucore::clap_localization::handle_clap_result(uu_app(), args)?; for all utils?
Then we don't spread many code.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/shuf/shuf-reservoir (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/sort/sort-stale-thread-mem (fails in this run but passes in the 'main' branch)
Note: The gnu test tests/basenc/bounded-memory is now being skipped but was previously passing.

[sylvestre]

what about we focus first on:

• GNU compat
• performance of critical programs like ls, cp, mv, sort, uniq, etc ?
• the backlog bugs
• more fuzzers
• code coverage
• ...

and come back on micro optimizations later?