CI status

[mihaimaruseac]

(This is more for me to know what the status is without reading all the TODOs and going back through the logs).

CI has been moved to GitHub actions as Travis is slow. There are 2 workflows, each in its own file: one for testing against Cabal and one for testing against Stack. In both cases, we use the latest toolchain (this means, we leave testing against Stack snapshots to the Stackage curator team).

There are a few remaining issues before a perfect CI:

• CI on Stack on MacOS is disabled because restoring from cache results in broken builds
• Testing is disabled on Windows, on both workflows as all tests raise NoFenceEnd
• Haddock documentation is only generated on Cabal workflow, as Stack wants to generate the documentation for all dependencies and this seems to break caching
• Caching on Stack uses two paths, only one of which works. Pending Add stack-root output actions/setup-haskell#35 we should be able to merge these into just one and have a good polyfill across operating systems
• Would be nice to have a continuous performance monitoring, but this requires more setup.