Commit generated protobuf code, only regenerate on-demand

[MCOfficer]

closes #41

Since v0.11 (4fea65b), prost-build requires protoc to be installed while compiling. This means any downstream application / library of bitar also needs it to compile.

This PR adds the generated code directly the repository, and only regenerates it on-demand.

Deciding when to regenerate the code is a bit of a pain - you can't use cargo:rerun-if-changed, since that will run directly after a clean check-out, and thus fail without protoc. The best compromise between elegance and practicality i found was to only regenerate when the generated file has been removed. I've added some comments to clarify this; since the file is rarely changed it shouldn't be too much of a hassle.

Alternative solutions to consider:

• use rerun-if-changed and check if protoc is in PATH. This "silent switch" could lead to build inconsistencies downstream, where the buildscript also runs.
• instead of shipping a .rs file, compile the protobuf files to a .bin using protoc and commit that. The buildscript would work as before (sourcing from the .bin instead of .proto3), without needing protoc. See for instance feat: vendor pbjson-types descriptor set (#62) influxdata/pbjson#79
• Ship protoc with this crate, despite the security concerns raised in Remove build.rs tokio-rs/prost#657. See protoc-prebuilt, protoc_bin_vendored, protoc_fetcher

[MCOfficer]

Should also remove protoc from the CI, but i want to hear your thoughts first.

[oll3]

Looks good! Added a single request/question.

Other than that I think the method you've selected seems like the best option given the alternatives you listed. I think it makes it simple enough to regenerate the file when wanted.

Should also remove protoc from the CI, but i want to hear your thoughts first.

The only reason I can think of to keep it would be to validate the checked-in file, but don't know what such a test would be good for really. Anyway, it can be restored if ever needed so let's remove it.

[oll3]

Thanks @MCOfficer ! ⭐