chore: encapsulate IO logic in IO functions

[TomAFrench]

Related issue(s)

Resolves #

Description

Summary of changes

We currently had some special logic related to:

• avoiding reading files which we expect to be empty
• splitting off return value from BTreeMap loaded from file / inserting return value to be saved to file.

which exists in the CLI business logic. As this is purely an IO implementation detail I've pushed it inside the relevant helpers.

Dependency additions / changes

Test additions / changes

Checklist

• I have tested the changes locally. (Tested and confirmed to work in conjunction with fix(abi): ensure that return value is loaded from toml #883)
• I have formatted the changes with Prettier and/or cargo fmt with default settings.
• I have linked this PR to the issue(s) that it resolves.
• I have reviewed the changes on GitHub, line by line.
• I have ensured all changes are covered in the description.
• This PR requires documentation updates when merged.

Additional context

[jfecher]

👍