Replace `NonNull<u8>` for raw messages with a dedicated opaque pointer

These are more type safe, and more clearly distinguish between a
raw message and serialized data.

This also defines a macro to create new opaque pointer types, and
switches `RawArena` to using it.

PiperOrigin-RevId: 552957136

Protobuf Team Bot committed 2y ago

1525300472ac3e0e3d17fc0451d1d0907700efcc

Parent: b09aa96

Committed by Copybara-Service <copybara-worker@google.com> on 8/1/2023, 11:23:16 PM