fix(p/int256): "-0" Output When Calculating Basic Arithmetics

[notJoon]

Description

• Added a check to prevent appending a minus sign to zero values. This ensures that "0" always returned for zero values.
• Fix negative number handling in the Arithmetic function
• Add divide by zero panic in Div function

Contributors' checklist...

• Added new tests, or not needed, or not feasible
• Provided an example (e.g. screenshot) to aid review or the PR is self-explanatory
• Updated the official documentation or not needed
• No breaking changes were made, or a BREAKING CHANGE: xxx message was included in the description
• Added references to related issues and PRs
• Provided any useful hints for running manual tests
• Added new benchmarks to generated graphs, if any. More info here.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 60.79%. Comparing base (701ac17) to head (4e84f64).
Report is 19 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2750      +/-   ##
==========================================
+ Coverage   60.21%   60.79%   +0.57%     
==========================================
  Files         562      563       +1     
  Lines       75038    75985     +947     
==========================================
+ Hits        45185    46192    +1007     
+ Misses      26476    26398      -78     
- Partials     3377     3395      +18     

Flag	Coverage Δ
contribs/gnodev	61.46% <ø> (+0.06%)	⬆️
contribs/gnofaucet	15.31% <ø> (+0.85%)	⬆️
gno.land	67.21% <ø> (+2.64%)	⬆️
gnovm	65.59% <ø> (+1.26%)	⬆️
misc/genstd	80.54% <ø> (ø)
misc/logos	20.23% <ø> (ø)
tm2	62.17% <ø> (+0.15%)	⬆️

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[deelawn]

Just one comment. Looks good to me other than that.

[thehowl]

I think there's a problem at the root of this package; which is why is this not using two's complement?

The correct name for this package would actually be int257, because each Int is actually storing 257 bits of information, seeing as we separate the sign from the actual number.

If you were using two's complement, you wouldn't have problems like -0 to begin with, and addition/subtraction would Just Work without using any conditionals. So, if you're looking for improvements on this package, that's one place to look.

Aside from that, approach looks good. I made a suggestion to organize/optimize the result of Cmp.

[notJoon]

I think there's a problem at the root of this package; which is why is this not using two's complement?

If my recall correctly, this topic came up during the implementation, but we decided to follow another implementation that we were referencing, so that we eneded up setting the neg field instead of two's complement.

I definitely agree that we can eliminate a bunch of codes in current implementation. I'm currently preparing for a improvement of the 256-types, and I'll add this change as part of those modifications.