Make oak_abi no_std compatible

[blaxill]

Currently oak/server/rust/oak_abi requires std as the generated protobuf files use std. The rust runtime is intended to be no_std compatible and so needs oak_abi to also be no_std compatible. The options available in approximate order of effort to benefit:

• Patch the generated generated protobuf files
• Patch the upstream protobuf generator to be no_std compatible
• Replace the protobuf files with manual instantiations (at the cost of losing auto-synchronisation for oak abi definitions)

[tiziano88]

Another alternative would be to instead define those constants in a .h file, and use rust-bindgen to generate Rust bindings for them (they would look like separate consts rather than actual enums though, which is not great).

[daviddrysdale]

No longer relevant – we're not targetting a no_std build of Rust code any more.