* Initial version of ci.yml

This is a straight copy from Dmitrii Kovanikov's blog post
at https://kodimensional.dev/github-actions.

Will adapt to haddock in successive commits.

* Delete .travis.yml

* Modify to only test on ghc-8.10.{1,2}

* Use actions/setup-haskell@v1.1.4

* Relax QuickCheck bound on haddock-api

* Remove stack matrix for now

* Nail down to ghc-8.10 branch for now

* Pin index state to 2020-12-08T20:13:44Z for now

* Disable macOS and Windows tests for now for speed up