Implement `SpacetimeType` for `Result<T, E>`

[gefjon]

Make it a new "special" type, like Option. Use ok and err (lower-case) as variant names for consistency with SATS Option. This will also require adding codegen support to all our client SDKs.

[gefjon]

Desirable because procedures return SpacetimeType values, and we'd like users to be able to return Result for error-handling.