simplify the std bench improvement candidate

[maxim-uvarov]

@amtoine gave me valuable feedback about the PR of std bench improvement CANDIDATE into stdlib-candidates.

I understood and respected his reasoning about his position. Yet, I still believe that users of std bench will benefit from my proposal.

I incorporated some of @amtoine advice in this PR.

I removed the bench --units option as I now believe it is better to encourage users to use core functionality for formatting duration (which I had not thought of initially) and to avoid multiplying --options.

bench -n 10 {sleep 0.1sec} | format duration ms mean min max stddev
# or
bench -n 10 {sleep 0.1sec} | format duration ms ...($in | columns)

Also, I removed the option of --sign-digits and just hard-coded the precision of conversion to the fourth significant digit (which will give a maximum relative error of 0.05%, which I still think is unnecessarily precise).

To explain my motivation, I added some context from our previous conversation:

Maybe you would agree with me that this representation is very wordy and unnecessary, even for a professional?
1sec 333ms 333µs 333ns

maybe, but then i would argue the issue comes from the formatting nushell does on durations, not from std bench, it's just that Nushell will show every part of the duration 😕

And I add here that if Nushell adds the setting for resetting insignificant digits from displaying, those conversions could be removed for the better. Yet, we don't have such a setting yet, but we already use bench and use it quite often. So, I propose this usability improvement.

In my defense, I would add that the existing --pretty option will only benefit from the proposed changes (while it can't benefit from | format duration ms).

> bench {sleep (10sec / 3)} --rounds 2 --pretty
- 3sec 335ms 974µs 264ns +/- 1ms 108µs 748ns
+ 3sec 335ms +/- 1ms 108µs

Finally, I kept the --list-timings option because I strongly believe that users much more often will not need expanded 50 lines of timing results on their screen (which they can get rid of by adding | reject time in the second execution of the bench command - but I would like to avoid this second execution).

I won't be hurt if my proposed changes aren't accepted and applied to mainstream. Yet, I feel like my initial PR is still valuable and will benefit from my current PR's additions.

[fdncred]

Thanks