Add GitHub actions

[andreasbuhr]

This pull request should add support for github actions. It is in a very early stage and should be significantly improved before merged.

Any help or review is greatly appreciated. Especially, help in creating MSVC tests would be great.

To help improve this pull request, please fork https://github.com/andreasbuhr/cppcoro/tree/add_github_actions and create a pull request toward this branch.

[andreasbuhr]

Starting to look nice, have a look here: https://github.com/andreasbuhr/cppcoro/runs/1245006472 Next step is to add MSVC.

[lewissbaker]

Feel free to steal bits from libunifex actions config.

https://github.com/facebookexperimental/libunifex/blob/master/.github/workflows/libunifex-ci.yml

[andreasbuhr]

This pull request is now open for two month. I now stop maintaining this pull request. From now on, I will continue to maintain the master branch in https://github.com/andreasbuhr/cppcoro. There we have CMake support, a CI based on Github Actions, and support for the latest compilers.