[internal] Continuous Integration on GitHub Actions

[François Best]

This issue tracks progress of moving continuous integration from TravisCI to GitHub Actions.

CI steps:

• Build the library code via CMake
• Build and run unit tests
• Generate code coverage and upload to Coveralls
• Use PlatformIO to build all examples with a variety of officially supported Arduino (and variants) boards (eg: Teensy, ESP32).
• Reduce checks verbosity for examples matrix
• Deduplicate checks for PRs (push + PR sync creates redundancy)

CI will run for each pushed set of commits on any branch, and on PRs.

A passing CI should be required on the master branch.