Add str conversion methods to std::string::String in sway-lib-std

[camden-smallwood]

Description

This adds string slice conversion support to the std::string::String struct in the Sway standard library through the String::as_str and equivalent impl From<str> for String and impl From<String> for str methods.

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).
  • If my change requires substantial documentation changes, I have requested support from the DevRel team
• 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.

[fuel-cla-bot]

Thanks for the contribution! Before we can merge this, we need @camden-smallwood to sign the Fuel Labs Contributor License Agreement.