signed u256 - i257

[shramee]

Pull Request type

Please check the type of change your PR introduces:

• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no API changes)
• Build-related changes
• Documentation content changes
• Other (please describe):

What is the current behavior?

We don't have signed u256.

Issue Number: N/A

What is the new behavior?

We have signed u256.

Does this introduce a breaking change?

• Yes
• No

Other information

The code and tests for this is largely taken from old i129 int. I've tried to make commits with old pasted code and tweaks made on that commit. Hopefully that will make reviewing easier.

Source: https://github.com/shramee/alexandria/blob/7e0f050b97c42b2a5916d552494d3724ae2c0197/alexandria/math/src/signed_integers.cairo#L1618

Tests come from: https://github.com/shramee/alexandria/blob/7e0f050b97c42b2a5916d552494d3724ae2c0197/alexandria/math/src/tests/signed_integers_test.cairo#L1279

[shramee]

I'm using i257 for Euclidean algorithm implementation of alexandria_math::mod_arithmetics.

https://x.com/shrameetweets/status/1743034431521124704?s=20

[0xLucqs]

revert the DivRem plz otherwise lgtm

[shramee]

@LucasLvy Done