Add u256 conversions

[SwayStar123]

Description

Adds conversion methods from u8, u16, u32, u64, b256 to u256, and u256 to b256

Closes #4800

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]

LGTM, but you need tests.

[K1-R1]

lgtm