Support for u256 constants

[xunilrj]

Description

This PR is part of #4794. It implements the bare minimum to support u256 constants across the stack.

They are represented in sway in hex literals with u256 suffixes. Hex without suffix will still be b256, for backward compatibility until we implement u256 completely. Then we can decide what to do.

There are multiple places in the code that will fail if the literal does not fit in u64. This will be fixed later in a specific PR for big u256. Some places have temporary unwrap(), that will be removed later. I can try to remove them now, if necessary.

I am leaving all documentation updates for when everything is implemented.

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.