Rather than requiring developers to write:
- a parallel Mojo struct
- Manually serialize/deserialize from string (which is slightly dubious, since Mojo strings are supposed to be valid UTF-8, so this would actually require then converting from string to array<uint8>...)
- Similar to the above but wrapping the string field in a dedicated struct for typemapping purposes

It would be convenient if we could say something like:
[EncodedAs=proto]
struct Foo;