rust/oak_abi: Make oak_abi

[blaxill]

This is a temporary fix to allow oak_abi no_std compilation. See #638, a follow up PR will be done to resolve this long term. I would like to add this as a temporary fix as it blocks work on making the oak_runtime no_std compatible.

Checklist

• Pull request affects core Oak functionality (e.g. runtime, SDK, ABI)
  • I have written tests that cover the code changes.
  • I have checked that these tests are run by Cloudbuild
  • I have updated documentation accordingly.
  • I have raised an issue to
    cover any TODOs and/or unfinished work.
• Pull request includes prototype/experimental work that is under
  construction.

[tiziano88]

Would something like this work?

#[cfg(feature = "no_std")]
mod inner {
  pub enum ChannelReadStatus { /* ... */ }
  pub enum OakStatus { /* ... */ }
}

#[cfg(feature = "std")]
mod inner {
  mod proto;
  pub use proto::oak_api::{ChannelReadStatus, OakStatus};
}

pub use inner::*;

[blaxill]

I can't put the mod proto; inside inner, but I can do the rest, WDYT?

[tiziano88]

LG! Are nested mods not supported then?

[blaxill]

They are supported, but as that one in particular is a file import, rustc looks for the definition in src/inner/proto and using super/crate doesn't seem to work for this.