feat(p/int256): Optimize int256 with two's complement implementation

[notJoon]

Description

This PR optimizes the implementation of int256 type. Key changes include:

• Changed from storing sign and value separately in the Int256 struct to an implementation using two's complement method.
• This reduces unnecessary operations and improves overall performance.

Performance Result

• Basic arithmetic operations (addition, subtraction, etc.): About 3x performance improvement (based on Go benchmarks, may differ slightly in gno)
• Division operations: Up to 5x performance decrease compared to the previous implementation (can be improved by directly manipulating array fields, but not applied to avoid duplication with p/demo/uint256)

Additional improvements:

• Increased test coverage to 95%.

This change is expected to improve performance for most int256 operations. However, please note the performance degradation in division operations.

See Also

#2750 (review)

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 60.91%. Comparing base (4f6ca96) to head (7053e0b).
Report is 10 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2846      +/-   ##
==========================================
- Coverage   60.91%   60.91%   -0.01%     
==========================================
  Files         564      564              
  Lines       75273    75273              
==========================================
- Hits        45851    45850       -1     
- Misses      26053    26054       +1     
  Partials     3369     3369              

Flag	Coverage Δ
contribs/gnodev	61.46% <ø> (ø)
contribs/gnofaucet	14.46% <ø> (-0.86%)	⬇️
gno.land	67.17% <ø> (ø)
gnovm	65.77% <ø> (ø)
misc/genstd	80.54% <ø> (ø)
misc/logos	19.88% <ø> (ø)
tm2	62.07% <ø> (+0.01%)	⬆️

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