Add tests to compare size of compiled byte code between changesets

[sohkai]

Not quite sure how we'd get the old compiled byte code to diff against the new one... but should be possible.

Could be done via a custom Github app, Danger (e.g. see how react's dangerfile does the git actions, or as part of a travis build.

[izqui]

I think something like Danger could be a great tool for this. I believe they even have integration with the new Github Checks API

[sohkai]

Partly done through #387.

Now we just have to figure out how to automate this...