feat: add U256 support

[hal3e]

closes: #1013

Adds support for sway's U256 type.

Checklist

• I have linked to any relevant issues.
• I have added tests that prove my fix is effective or that my feature works.
• I have added necessary labels.
• I have done my best to ensure that my PR adheres to the Fuel Labs Code Review Standards.
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[segfault-magnet]

The JSON serialization of U256 seems a bit unexpected.

Take the following example:

        let num: U256 = U256::from(123);
        let serialized: String = serde_json::to_string(&num).unwrap();
        eprintln!("{num} serialized is: '{serialized}'");

        let num: u128 = 123;
        let serialized: String = serde_json::to_string(&num).unwrap();
        eprintln!("{num} serialized is: '{serialized}'");

Which prints out:

123 serialized is: '"0x7b"'
123 serialized is: '123'

The U256 is serialized as hex while u128 is a stringified decimal number. Might take somebody by surprise.

Although it is probably faster to do the hex dump, I'd sacrifice the speed and go with the least surprising approach.

[Salka1988]

👍

[iqdecay]

Perfect on the first try!