Fix containing StorageStrings in StorageVecs

[ironcev]

Description

This PR fixes #6036 by using field_id() in the StorageString instead of slot().

Additionally, the PR:

• fixes documentation glitches in the StorageVec.
• optimizes StorageMap::get by removing double calculation of the key.
• cleans up most of the Sway and all Rust warnings in the SDK harness tests.

The PR is not solving all the issues related to composing dynamic storage types. E.g, #6040 is (very likely just one) of them.

Closes #6036.

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.

[bitzoic]

🎉