_This change should have no user impact._ And you can see that the only
changes to generated files are in comments.
Before this change, we used comments to denote reserved or deprecated
fields. This works, but I think we should instead use the `reserved`
keyword, which offers a few advantages. From [the protobuf docs][0]:
> If you update a message type by entirely removing a field, or
> commenting it out, future users can reuse the field number when making
> their own updates to the type. This can cause severe issues if they
> later load old versions of the same .proto, including data corruption,
> privacy bugs, and so on. One way to make sure this doesn't happen is
> to specify that the field numbers (and/or names, which can also cause
> issues for JSON serialization) of your deleted fields are reserved.
> The protocol buffer compiler will complain if any future users try to
> use these field identifiers.
This updates our proto files to use `reserved` instead of comments. It
also adds support to our wrapper script. (I moved a few things around in
that script, too, for consistency.)
I think this is a useful change on its own, but I think it'll make
things a little better when we deprecate some fields, which we're
planning to do soon.
[0]: https://developers.google.com/protocol-buffers/docs/proto3#reserved
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