Bundle-size delta check/message on PR

[LesnyRumcajs]

Based on a discussion with @anorth in #497 it would be beneficial to include the bundle size delta between master and candidate branch in a PR message.

Some changes are clearly increasing bundle size, but some of them are a bit sneaky (e.g. bumping a dependency that by default would now include a large set of unused features). It would be great to not have to have it at the back of the head but right in front of us.

I'd imagine a GH bot posting a message to PR (or modifying an existing one to not bloat the discussion) with a simple cargo clean && cargo build && make bundle && du -b output/builtin-actors.car on both branches, calculating the delta in bytes and %.

Probably something along the lines of this workflow would do the trick (that is to say, having a modifiable message with shell output): https:/LesnyRumcajs/grpc_bench/blob/master/.github/workflows/issue_bench.yml

[anorth]

Thanks!

An alternative implementation is a build step that writes the output size to a file that's checked in to git (orchestrate from Makefile or similar). In CI, build should check that the calculated value equals the written value. This puts the delta into code review directly rather than a comment on the GitHub issue, and makes the history of sizes easy to see.

Either could work,

[jennijuju]

@LesnyRumcajs would you be open to take a stab on this one?

[LesnyRumcajs]

@jennijuju Sure thing, I'll look into this next week.