chore: move nargo I/O into separate module

[TomAFrench]

Related issue(s)

Resolves #884

Description

Summary of changes

The majority of the changes are just moving functions which load/write to file into the new fs module. I've also removed the preprocessing logic from fetch_pk_and_vk so that it's pure I/O as it's the CLI's responsibility to perform preprocessing where necessary.

Dependency additions / changes

Test additions / changes

Checklist

• I have tested the changes locally.
• 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

[phated]

Do we want to take this further and split into the nargo-core and nargo-cli crates like @kevaundray suggested? I could see nargo-core becoming the basis for wasm stuff instead of the special noir_wasm crate.

[jfecher]

I'm fine with splitting out nargo-cli into a separate crate if it means less code duplication for the wasm crate, but this should be in a separate PR.

[TomAFrench]

I'm aiming to not change functionality as much as possible in this PR so that substantive changes can be reviewed separately from code reshuffling.

That said, I also think that we should wait until nargo sits in its own repository before we split it up based on how small the resulting crates would be.

[TomAFrench]

Will merge after #892