Goodbye Appveyor, Hello Github Actions

Frank Quinn

Hi folks,

I am in the final stages of the effort to migrate from Appveyor to Github Actions. This will reduce the overall build time down from 5 hours to 40 minutes across 14 different development environments including new support for OSX in continuous integration. It also provides much richer options around bringing our own compute should we need to with better support for parallel jobs.

These changes will also (finally) retire the Scons build system so if you are still using it, please use cmake instead as documented here:

This will be dropping into the "next" branch at some point over the weekend if anyone finds any issues please let me know!