API-compatibility with the original one is retained.
Now that we don't need to keep code parity with core, we could do some
changes that make life easier.
All generated classes are now in one file. This makes github diffs more
readable, at the cost of somewhat complicating inspecting individual
classes; however, that is something we shouldn't be doing anyway.
Enums are now implemented as enum.IntEnum.
The original class-level FIELDS member was restored.
Each field is now defined via protobuf.Field, which is easier to work
with in the codec, AND we're not stuffing defaults and flags into the
same field.
