u256 more operations

[xunilrj]

Description

This PR is part of #4794 and implements more operators for u256:

• sub
• mul
• div
• comparisions
• mod

Checklist

• I have linked to any relevant issues.
• I have commented my code, particularly in hard-to-understand areas.
• I have updated the documentation where relevant (API docs, the reference, and the Sway book).
• I have added tests that prove my fix is effective or that my feature works.
• I have added (or requested a maintainer to add) the necessary Breaking* or New Feature labels where relevant.
• I have done my best to ensure that my PR adheres to the Fuel Labs Code Review Standards.
• I have requested a review from the relevant team or maintainers.

[IGI-111]

Just some nitpicks, otherwise LGTM

[vaivaswatha]

We need IR parsing tests for the new IR opcodes (for this and the recently introduced ones too). I'm guessing that they can go into sway-ir/tests/serialize.

Otherwise LGTM.

[xunilrj]

We need IR parsing tests for the new IR opcodes (for this and the recently introduced ones too). I'm guessing that they can go into sway-ir/tests/serialize.

Done.