chore: document u128 feature

[guipublic]

Description

Problem*

Resolves #4142

Summary*

Add documentation for U128

Additional Context

Documentation*

Check one:

• No documentation needed.
• Documentation included in this PR.
• [Exceptional Case] Documentation to be submitted in a separate PR.

PR Checklist*

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt on default settings.

[github-actions]

🚀 Deployed on https://65c270e458b37b1a42b0b029--noir-docs.netlify.app

[Savio-Sou]

cc @noir-lang/developerrelations re doc changes.

[signorecello]

Just a question here @guipublic don't all casts to unsigned integers have a performance cost? I was under that impression (because range checks).

Is this cost simply higher than usual? If not, we should maybe point to the performance cost at the top of the page and remove this one to avoid confusion

[guipublic]

There is no cast for U128, as explained in the doc, so I am not sure to understand your question.
In term of performance, converting an integer to U128 will have no cost (because U128 is the biggest), and converting to a lower bit size will not have a specific drawback because of U128, just what you should expect.

[Savio-Sou]

I guess what @signorecello meant was working with native uints is less performant than working with Fields.

This section reads like performance costs would rank as U128 > native uint > Field.
Is that the correct way to understand it?

[guipublic]

Yes, the higher the bit size, the higher is the cost. But because U128 uses 2 limbs, cost of arithmetic operations are even higher, especially for multiplication.