deprecated attribute

[xunilrj]

Description

This PR is part of #4794. It is the minimum implementation for deprecation. Pretty much only what was needed for deprecating U256.

It also minimally implements #[allow(deprecated)].

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.

[xunilrj]

Reminder to ping #4779 when this is merged.

[ironcev]

Nice minimal implementation of deprecated! Just a few minor remarks from my side but otherwise LGTM

[ironcev]

Since the UsingDepricated warning is brand new providing Diagnostic for it from the beginning will save us effort down the road. @xunilrj If you are find with that I can add it.

[xunilrj]

Since the UsingDepricated warning is brand new providing Diagnostic for it from the beginning will save us effort down the road. @xunilrj If you are find with that I can add it.

I can do it. Do we have a guide I can follow?
I will merge this PR and create a new one, just to finish this.

[ironcev]

I can do it. Do we have a guide I can follow?

Great thanks! The guidelines are at the moment only in the RFC: https://github.com/FuelLabs/sway-rfcs/blob/master/rfcs/0011-expressive-diagnostics.md#wording-guidelines

I'll bring them into the sway-error README.md and expand with examples.

It is also good to take existing warnings in the impl ToDiagnostics as examples.

Let me know if you have questions. We will all anyhow need a bit of practice there 😄