Investigate making rust-protobuf no_std compatible

[blaxill]

It should be possible to make rust-protobuf no_std compatible. There are different approaches we could take either:

• Use no-std-compat or similar to compile rust-protobuf to no_std. This would potentially be the least amount of work, but likely would require vendoring rust-protobuf under third_party/ and re-adding the std::prelude import to all the files (as no_std removes the implicit prelude import)
• Edit all files to use alloc / core and fix any remaining issues manually. Hopefully as automated as possible.

If it is not viable to move rust-protobuf to no_std, then we may be required to move to a different library such as prost.

[daviddrysdale]

We moved to Prost everywhere in #829, and away from Asylo in #709 – is this still relevant?