Use GitHub Actions for continous integration

[mdpiper]

This PR replaces our current Travis CI workflow with GitHub Actions.

Features:

• Runs on Linux and macOS, on py38 and py39
• Uses mamba to install dependencies into a test conda environment
• Checks out, builds, and installs the four BMI language examples, then runs tests on them
• Lints once, on Linux with py38
• Builds docs on Linux with py38

Windows is not tested--we already have a working solution with Appveyor. I'll add an issue recommending we fold the CI on Windows into Actions, as well.

[mdpiper]

For reference, our Travis CI workflow is preserved in the reference/ci-with-travis branch.