Haskell CI via GitHub Actions

[andreasabel]

Add GitHub Actions generated by haskell-ci.

This runs build/tests on github for all ghc versions in field tested-with. (Currently 8.0 - 8.10 latest minor versions.)
(Does not do any hpack-specific things, should there be any in the other CIs: travis, appveyor.)

Rationale: alternative to Travis CI.

[andreasabel]

Breaks appveyor:

The file C:\projects\hpack\package.yaml requires version 0.34.3 of the Hpack package specification, however this version of stack only supports versions up to 0.31.2. Upgrading to the latest version of stack may resolve this issue.

Please activate "Actions" to get the new actions in this PR running.

On my account, actions succeed for ghc 8.8 and 8.10 but fail for ghc <= 8.6 because of the mismatch between installed Cabal version and Cabal version required by hpack. I filed haskell-CI/haskell-ci#468.
The quick workaround would be to disable these CI instances, only keeping 8.8 and 8.10, but maybe there is a better solution: fixing the script somehow or a fix of haskell-ci upstream.

[sol]

@andreasabel Hey! Thanks for working on this! I have been looking into GitHub Actions myself lately. #423 is what I came up with + what use I across my repos. Specifically, I want the finally success job so that I can require it in a branch protection rule. This allows me to use GitHub's awesome new auto-merge (beta) feature.

[sol]

Closing in favor of #423.

[andreasabel]

I think you should salvage this hunk from my PR.

[andreasabel]

Fair enough. But I think you should add the tested-with from my PR to hpack.yaml.