Bundle-size delta check/message on PR

Based on a discussion with @anorth in https://github.com/filecoin-project/builtin-actors/pull/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://github.com/LesnyRumcajs/grpc_bench/blob/master/.github/workflows/issue_bench.yml