GitHub actions

[AnderEnder]

Description of changes:

• This adds GitHub actions for CI, which can replace TravisCI. A continuation of initial PR#155.
• additionally replace deprecated usage of traits with dyn to fix the CI
• additionally add fmt changes to fix the CI
• nightly was added but temporary disabled because there is no direct analogue of travis ALLOW_FAILURES=1 feature

By submitting this pull request

• I confirm that my contribution is made under the terms of the Apache 2.0 license.
• I confirm that I've made a best effort attempt to update all relevant documentation.

[davidbarsky]

Thanks for doing this! I'm happy to merge this as-is, but I'm happy to accept failures on nightly for the time being. I can merge as-is, enabling nightly in a followup PR, or you can enable nightly now—your choice!

[AnderEnder]

I can merge as-is, enabling nightly in a followup PR, or you can enable nightly now—your choice!

@davidbarsky. I have not found a proper way to support nightly with failures yet(my current ideas are quite ugly). So my proposition is to merge it as-is. I am continuing my research to find a proper way or maybe someone else will propose it in another PR.