Implement Iterator for StorageVec

[ironcev]

Description

This PR implements Iterator for StorageVec.

Iterating over a StorageVec via for loop should now be the preferred option. Compared to the traversal via while loop and incrementing index, the for loop is:

• more performant. for loop eliminates the redundant boundary checks done in every get() call in the while loop.
• more idiomatic and convenient.

Closes #6796.

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.