expr: Get rid of clap for performance reasons

[RenjiSann]

As a side effect, we also now have the right behavior when adding arguments after --help or --version. Before this change, clap would have printed the help, but it is expected that flags are ignored if argc > 2 (more than 1 arg is given)

Benchmarks:

$ export N=2500

# Before
$ hyperfine -m 1000 -- "./expr_clap $(yes 1 | head -n $N | paste -s - 2>&1 | sed 's/\t/ + /g')"
  Time (mean ± σ):      47.4 ms ±   3.3 ms    [User: 34.4 ms, System: 12.8 ms]
  Range (min … max):    37.3 ms …  74.4 ms    1000 runs

#After
$ hyperfine -m 1000 -- "./expr_no_clap $(yes 1 | head -n $N | paste -s - 2>&1 | sed 's/\t/ + /g')"
  Time (mean ± σ):      24.7 ms ±   2.2 ms    [User: 13.9 ms, System: 10.7 ms]
  Range (min … max):    18.8 ms …  34.7 ms    1000 runs

[tertsdiepraam]

Cool! Here's a small suggestion

[github-actions]

GNU testsuite comparison:

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

[RenjiSann]

by running the GNU tests manually, I realized that I forgot to handle -- which can appear in the command. I made the necessary changes (diff)

[github-actions]

GNU testsuite comparison:

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

[github-actions]

GNU testsuite comparison:

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