The biggest difference in the API is that fields are now public. The
exception from that is `oneof` fields, which still require setters and
getters.
I couldn't measure any difference in performance. I didn't expect any
difference either, but it's good that it didn't seem to regress. I
timed `jj debug operation <some hash prefix>`, which will read the
whole operation log (to check that the prefix is unambiguous).
I had tried to generate the protobuf code at build time many months
ago, but decided against it because it slowed down the build too
much. I didn't realize there was the
"cargo:rerun-if-changed=<filename>" feature that time. Given that that
exists, it seems like an obvious win to generate the source code at
build time.
I put the generated sources in `$OUT_DIR` (where [1] says they should
be), then include them in the `protos` module by using the `include!`
macro. The biggest problem with that is that I couldn't get IntelliJ
to understand it, even after enabling the experimental features
described in [2].
[1] https://doc.rust-lang.org/cargo/reference/build-script-examples.html#code-generation
[2] https://github.com/intellij-rust/intellij-rust/issues/1908#issuecomment-592773865
