- Implement RawFieldIterator and ProtoAccessor::raw_fields that yield
(field_number, raw_bytes) pairs for each field
- Extend Builder with per-field _written flags and add a with()
method to copy unseen fields from a source message
- Add ProtoBuilder::write_raw to copy pre-encoded field bytes
- Add tests for raw-field iteration, verbatim copying, and with()
Add `write_enum` and `write_message` to generate Rust enums and structs
from protobuf descriptors. Update the generator tests to support the
updated `generate_rust_code` signature.