Better compatability checks

[tombentley]

When you're releasing version X.Y+1 on Herd it would be great if it could compare the uploaded .car to the .car for X.Y and warn you if the two were not compatible.

I've raised this against Herd, but obviously finding out that you've broken bc at the time you're wanting to release a module is a little late, so a way for people to know sooner would be good (such as ceylon/ceylon-ide-eclipse#219, but perhaps we need a command line tool which can do that same thing)