Promote protobuf warnings to errors

[antiguru]

Protobuf warnings are currently silently swallowed by our build system. We can instruct protobuf to promote all warnings to errors, which will fail the build in the presence of protobuf warnings. This can be enabled for example like this:

diff --git a/src/repr/build.rs b/src/repr/build.rs
index 391fcafc7..7d6754962 100644
--- a/src/repr/build.rs
+++ b/src/repr/build.rs
@@ -13,6 +13,7 @@ fn main() {
     env::set_var("PROTOC", protobuf_src::protoc());

     prost_build::Config::new()
+        .protoc_arg("--fatal_warnings")
         .compile_protos(
             &[
                 "repr/src/antichain.proto",

Add the --fatal_warnings flag to all prost_build calls and fix the resulting errors.

[benesch]

Omg! You have no idea how many years it took me to make this a thing. protocolbuffers/protobuf#8131 (comment)