Refactor read and write access to namespace modules.

[tritao]

Description

This PR prepares existing code that deals with (mutable) access to namespace modules to work with an upcoming PR that introduces a module ID for optimization purposes.

It also refactors the ABI code to pass engines around, which is necessary for the same PR.

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.

[jjcnn]

LGTM